People continue to mature and grow as individuals throughout our lifetimes (at least ideally). So the 45 year old you would probably not only be better equipped to make decisions than the 25 year old version, but also in all likelihood be a very different person, both literally (in the replacing of your body’s physical makeup in that gap) and mentally. Nevertheless, the 45 year old version of you and the much less mature and less accomplished 25 year old version are, in most societies, living under the exact same set of rules and restrictions. So why is some arbitrary number like 18 or 21 considered the cutoff point when society says everyone should be operating on a level playing field, legally speaking? What is it that makes a person an “adult” anyway? Is it the ability to drive a car without supervision? Consent to intimate relations? Ability to cast a vote? Serve in the military? Buy a beer? And at what age is a person really ready to handle these responsibilities? These are all difficult questions that don’t lend themselves to easy answers.

For example, at just 16 years old, Alexander the Great was busy conquering Maedi, when they dared revolt against Macedonia; also at 16, a peasant girl by the name of Jeanne d’Arc was taking her first steps into historical prominence by having the gall to approach a garrison commander to tell him how to do his job. At 15, one Charles Algernon Parsons was busy inventing the precursor to the modern automobile.  At 16, Julius Caesar was heading his family after his father’s death. History is littered with individuals accomplishing remarkable “adult” things all below the age most countries would today say that they were sufficiently mature enough to be considered an adult. Unsurprisingly, for parts of history, the issue of when someone was ready to take over various adult activities largely fell to their family to decide.

Nevertheless, while maturity levels vary greatly from person to person and the people who know the individual best may be better able to determine things for everyday activities, a number that generally fits everyone is often needed to set certain communal laws. Thus, the age when a person is considered sufficiently able to manage these adult activities, and answer for others, is typically set by civil policy makers within a nation, and in certain cases, sometimes by each individual state within a country. For example, in California the age of consent is set at 18, a number largely proliferated by Hollywood and thus often considered the set age of consent in the United States. However, most states actually set it lower, and in many cases without restriction on age gap between the individuals after that cutoff; the important thing is simply consent.

As for a general age most developed countries of the world consider someone an adult, this is usually (somewhat arbitrarily) set at 18.  Exceptions include countries like Iran and Saudi Arabia where 15 is the age of majority, Cuba where it’s 16, and North Korea where it’s 17. Speaking from a religious standpoint, historically 13 for males and 12 for females was considered the age of majority in Judaism- essentially aligning with when most in said genders would have become reproductively mature.

Seemingly reflecting that just because someone has become sexually mature, doesn’t mean they’re necessarily mentally mature enough to be actually considered an adult, most cultures today choose a greater age of majority.

Today we know that the human brain doesn’t finish fully developing until closer to one’s mid-20s and even potentially into the 30s. (Similar to the earlier onset of puberty, the female brain typically finishes its development about two years before the male.) In fact, the frontal lobe, which is extremely important in our decision making, among other things including being critical for appreciating future consequences to current actions and inhibiting impulses, isn’t finished developing until many years after pretty much every culture in the world considers you an adult. Speaking of this, Neuroscientist Dr. Sandra Aamodt, notes,

One of the side effects of these changes in the reward system is that adolescents and young adults become much more sensitive to peer pressure than they were earlier or will be as adults.  So, for instance, a 20 year old is 50% more likely to do something risky if two friends are watching than if he’s alone.

So, today if we were most concerned about one’s mental maturity via underlying physical brain development being maximized before considering someone an adult, we might set the number at around 25-30 years old. But long before 25, most are sufficiently mentally mature enough and have enough base knowledge and experience to be out on their own in the world, even if they’re more susceptible to making poor decisions for themselves than they will be a few years later. This is likely why few have any problem with setting the number at something like 18 or 21.  As for how that specific number was reached… mostly arbitrarily.

For instance, the United States has long borrowed its original age of majority from British common law, which was set at 21. To quote:

By the common law the age of majority is fixed at twenty-one years for both sexes, and, in the absence of any statute to the contrary, every person under that age, whether male or female, is an infant.

Why this was the British common law age isn’t fully clear. One claim is that this derived from when squires could become knights.  However, if this is true, this seems more of a corollary, rather than a derivation, in that this is essentially when the squires were being considered “adults”. On top of that, there are certainly many known exceptions where individuals were considered knights before this age. It would seem more likely that this was simply the age around which it was generally observed that people started to reach sufficient enough mental maturity to no longer need any type of direct supervision.

It should also be noted that it’s only been an extremely recent phenomenon that the age of majority was given much credence to in many legal scenarios.  For instance, historically teenagers could, and sometimes were, members of British Parliament, and even exceptionally young children (around kindergarten age today) could sign contracts that were legally binding, such as contracts that compelled them to work for some individual or business even into adulthood. Children could also be executed or face other extremely serious consequences for crimes they committed.

After the initial adoption of 21 years old in the United States, the age of majority waffled over the decades, varying from activity to activity and state to state. For instance, on a national level, prior to 1971, the voting age was still 21, but then was lowered to age 18 by the 26th Amendment.

What spurred a change and why 18? As for the number, this seemingly was at least partially chosen because that’s the age when most people graduated high school, and the majority of people in the United States attended such a public institution when the switch was made. As for why the age was changed at all, the reasons behind this include massive improvements in public education since the 21 number was originally set, recognition that 18 year olds were able to enter into legally binding contracts, get married, have children, and could even be forcibly drafted.

This latter point was critical to the change as 18 year old males at this time could be forced to fight in a war, but had no say in whether their country went to war in the first place. This was a particularly hot topic in the Vietnam War era where many youth strongly opposed the war and those politicians who felt likewise were keen on getting those young people the ability to vote. In the contemporary words of Senator Edward Kennedy,

First, our young people today are far better equipped — intellectually, physically, and emotionally — to make the type of choices involved in voting than were past generations of youth. Many experts believe that today’s 18 year-old is at least the equal, physically and mentally, of a 21 year-old of his father’s generation, or a 25 year-old of his grandfather’s generation.

The contrast is clear in the case of education. In 1920, just fifty years ago, only 17% of Americans between the ages of 18 and 21 were high school graduates. Only 8% went on to college. Today, by contrast 79% of Americans in this age group are high school graduates. 47% go on to college…

Because of the enormous impact of modern communications, especially television, our youth are extremely well informed on all the crucial issues of our time, foreign and domestic, national and local, urban and rural…

Our 18 year-olds today are a great deal more mature and more sophisticated than former generations at the same stage of development. Their role in issues like civil rights, Vietnam and the environment is as current as today’s headlines. Through their active social involvement and their participation in programs like the Peace Corps and Vista, our youth have taken the lead on many important questions at home and overseas. In hundreds of respects, they have set a far-reaching example of insight and commitment for us to emulate…

Second, by lowering the voting age to 18, we will encourage civic responsibility at an earlier age, and thereby promote lasting social involvement and political participation for our youth.

The Amendment was passed with overwhelming support under the slogan “old enough to fight, old enough to vote.”

On the other hand, also in the 1970s, several states raised their minimum legal drinking ages above 18. This culminated in the National Minimum Drinking Age Act (1984), which penalized states for allowing people under 21 to purchase and publicly have alcohol. Apparently- old enough to kill for one’s country, but not old enough to imbibe.

In truth, over consumption of alcohol has potentially much more detrimental effects on developing brains than matured ones and, perhaps further reflecting the aforementioned research on physical brain development well beyond 18, particularly with regards to decision making, the switch from 18 to 21 on the legal drinking age was credited with reducing traffic fatalities by 20% shortly after the law was changed. Of other developed countries, however, only South Korea, Japan and Iceland also have a minimum drinking age over 18 and their older teens seem to handle it just fine.

As for age of consent, in the Western world (and generally speaking) before imperial Rome, girls were deemed sufficiently mature for marriage and sex when they first started menstruation (and boys, by the way, when they developed pubic hair).

But, note, as with some of the later laws we’re going to talk about, this had much more to do with guidelines around marriage, rather than actually receiving anyone’s literal consent for sex.  In addition, marriage was relatively unregulated by the state then, and instead was largely seen as a private family matter, so it is presumed these boundaries were flexible.

Moving on, the Catholic Church had rules for just about everything during the Middle Ages, and one of its most authoritative texts was the Decretum Gratiani. Written by the jurist Johannes Gratian in the 12^th century, it set a minimum age for betrothal (not necessarily marriage) at seven years for boys and girls, and the lawful age for a woman to consent to marriage (and “carnal intercourse”) at twelve, although certain unusual circumstances would render marriages at younger ages valid as well.

Essentially, the general opinion through much of human history was that around that age humans become sexually mature, at least speaking reproductively, as with animals, they didn’t see any point in restricting people from marrying and reproducing, despite the mental immaturity.

All that said, while these numbers are shockingly low by our modern sensibilities, contrary to popular belief, it has never really been the norm for such young individuals to get married, not even concerning women.  For example, in Massachusetts, records dating from 1652 to 1800 demonstrate that the average age of first marriage for ladies was between 19.5 and 22.5 years, and records for other colonies reflect similar ages. In fact, the average age of first marriage for all of the colonies studied was 19.8 before 1700, 21.2 during the early 18^th century, and 22.7 during the late 18^th century. Data gathered in England, France and Germany puts the average age of first marriage for women at a whopping 25.1 from 1750-1799 and 25.7 from 1800-1849.

By the end of the 19^th century, the average age when women were first getting married in the United States was between 22 and 24 years old, and this trend continued into the 1940s.

Surprisingly, the lowest average age of first marriage since the early 1700s was had by the baby boom generation, where the age dropped to 20.5 years. It is, perhaps, not coincidental that divorce rates peaked in the United States around the 1970s and early 1980s, and have been in strong decline ever since almost in lockstep with the average age of first marriage increasing- the more mature you are and the more you know yourself and where you’re going in life, in the aggregate the better decision you’ll probably make in choosing a life partner.

In any event, while one might technically be able to reproduce at ages around 12, occasional exploitation of such young individuals eventually spurred changes in the laws. As such, over centuries this age has gradually increased in most regions of the world. However, even today in some developed countries, the age to consent to sex remains below 16. In others, there is something of a sliding scale, in the United States deemed “Romeo and Juliet laws.” In these, it’s generally set that as long as two people are within a certain age-span of one another, it is legal for them to have consensual sex.

For example, in Washington State, it’s legal for a person as young as 11 to have consensual sex, so long as the person they are having sex with is at least 11 and not more than two years older than themselves. If even one day outside of that two years, it’s illegal and the older individual faces exceptionally stiff penalties, including up to life in prison in some cases.  Moving up the sliding scale, minors who are 12-13 are legally allowed to have consensual sex with someone as much as three years older than themselves and 14-15 year olds with someone who is no more than four years older than themselves.

Opponents of such systems argue that individuals that young can never actually give consent, as they are relatively easily coerced and not mentally mature enough to fully appreciate and factor in the potential ramifications of such acts (i.e. potential children and disease resulting from the act)- a position our current knowledge of brain development at least partially supports.

On the other hand, proponents of these types of laws and keeping the age of consent relatively low cite recognition that teenagers will frequently engage in consensual intimate activities with other teens, whether legal in their particular region or not. So potentially sending them to jail under some of the harshest penalties most legal systems have to offer for having consensual sex is absurd, particularly given that potential coercion from people far older, which is largely what originally spurred increases in age of consent, is comparatively rare, and in many cases would already be illegal anyway under separate laws.

On that note, in most jurisdictions, if two people are legally married, age of consent laws usually don’t apply at all.

In the end, beyond people maturing at different rates, even deciding what makes a person an adult at all is an amazingly complex question, let alone trying to decide a hard number where the transition takes place (in general) and what activities should be restricted only to people who’ve passed this point in age. But given the necessity to have a number for certain laws- in recognition that a poor decision in one’s youth shouldn’t necessarily ruin one’s entire life and children need certain extra protections and restrictions- most governments see fit to come up with hard numbers. Unsurprisingly given the complexity of the issues involved, while there is some loose rational behind the numbers, they have, on the whole, been set somewhat arbitrarily.

So why are you considered an adult in most of the world when you hit 18?  To sum up, people have long seemed to figure this was around the age when most humans reach a level of maturity and base-knowledge to be fully responsible for themselves and their own actions, even if said individuals will be far more mature and better equipped to handle the challenges of adulthood even just a decade later. That will always be true no matter what age you are if you’re continually improving yourself. But a baseline was needed, so a number was picked for certain official purposes, even if it isn’t perfectly ideal in all cases.

If you liked this article, you might also enjoy our new popular podcast, The BrainFood Show (iTunes, Spotify, Google Play Music, Feed), as well as:

• Why the Standard Retirement Age Is Set Where It’s At
• When Did Teen Girls Stop Commonly Getting Married?
• Why Do We Age?
• The Surprising Truth About Cousins and Marriage
• Do Your Ears And Nose Continue To Grow As You Age?