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Most countries define the age of adulthood at 18 years.

Why is this? Was there some historical precedent set in ancient Rome/Greece or something like that? Why 18 years and not some "nicer" number like 20 or 15?

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    I doubt that different countries set age of majority due to some historic precedence because there is no agreement on age and age of majority in ancient time was much lower than it is today. Age of majority ranges from 15 to 21 in different countries which is determined due to cultural/religious norms of the country in question. – NSNoob May 16 '16 at 8:30
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    Age of majority can vary within one country as well. For example in US, Age of majority varies from 16 to 21. In UK, Scotland has age of majority set to 16 while England has it set to 18. – NSNoob May 16 '16 at 8:33
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    Check the complicated table of age of majority at en.wikipedia.org/wiki/Age_of_majority - indeed, 18 is the most frequent one now but it's far from universal. In the Czech lands, for example, we've had age of majority between 14 and 24 years at different moments - cs.wikipedia.org/wiki/Zletilost#Historie - and in the ancient times, the age of majority depended on the aristocratic status or class (higher aristocrats were generally younger when recognized as adults - because they were learning more intensely as kids). – Luboš Motl May 16 '16 at 11:09
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    "Most countries, if not all, define the age of adulthood at 18 years." Reference? – user69715 May 16 '16 at 16:32
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    Even though not all countries have it, 18 is by far the most common. So I guess it still is an interesting question. – taninamdar May 16 '16 at 21:23
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+50

Age of consent laws could, in part, explain why we've settled on 18 or ages near to it. Age of consent statutes can be dated as far back as 1275 in England and were adopted in a number of other countries throughout Europe. Some of the first interpretations settled on the "age of marriage", which at the time was 12. Part of the problem with having a set number that determines the age of consent is explained by Robertson:

In trials, juries were often unwilling to simply enforce the law. Rather than focusing strictly on age, they made judgments about whether the appearance and behavior of a girl fit their notions of a child and a victim. It was not only that relying solely on age seemed arbitrary to them; at least until the end of the 19th century, age had limited salience in other aspects of daily life. Laws and regulations based on age were uncommon until the 19th century, and consequently so was possession of proof of age or even knowledge of a precise date of birth.

Once it became possible to reliable check the age of a person, the laws demarcating age became easier to enforce. Robertson continues:

At the end of 19th century, moral reformers drew the age of consent into campaigns against prostitution. Revelations of child prostitution were central to those campaigns, a situation that resulted, reformers argued, from men taking advantage of the innocence of girls just over the age of consent. The outcry it provoked pushed British legislators to raise the age of consent to 16 years, and stirred reformers in the U.S, such as the Women's Christian Temperance Union, the British Empire, and Europe to push for similar legislation. By 1920, Anglo-American legislators had responded by increasing the age of consent to 16 years, and even as high as 18 years.

What followed in the 20th century was a debate on what constitutes physiological, and psychological maturity. No doubt that this and other factors such as: compulsory schooling, mandatory military service, liquor laws, and the rise of higher education as a intermediary between adolescence and adulthood, played a role in us collectively as a society deciding that someone is an adult at 18.

Source: Stephen Robertson, "Age of Consent Laws," in Children and Youth in History, Item #230, https://chnm.gmu.edu/cyh/case-studies/230 (accessed May 26, 2016).

Edit

This paper by T.E. James (published in 1960 so it is a bit dated) suggests that common law settled on 21 as the age of adulthood. Common law was influenced heavily by Roman and Justinian laws, though these generally settled on 15 as the age of majority. James write:

In the absence of any clear authority, it may be assumed that at one time between the ninth and tenth century, fifteen was also the age of majority in England; but, by the time of the Magna Carta, this age had been raised to twenty-one, at least so far as men holding in knight service were concerned.

I think some have mentioned that age might have been linked to the increased weight of armor. I think there seems to be some truth in that. The fact that James writes in 1960 that the age of majority is 21 and that it has been that way for centuries (at least in England and some other European countries) lends itself to the argument that 18 might be a relatively recent age change.

  • Bureaucrats and officers of the law like clear-cut, objective criteria -- so birth certificates, various identification laws, and the use of age as the discriminating factor become interconnected as we delegate decisions to officialdom. As you pointed out in this answer, activists found this to be a great lever to achieve their goals. "Age of majority" has become the catch-all for all questions in this modern world. – Peter Diehr May 27 '16 at 20:29
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For a long time, 21 was the "age of adulthood". From Encyclopedia of Adult Development:

As early as the thirteenth century, age 21 was an important marker in Britain, at least for men. This was the age, for example, when they could begin to serve as knights. It was believed that the physical requirements of combat - to wear a heavy suit of armor and simultaneously lift a sword or lance - could only be met by men who had attained age 21.

It wasn't until the 26th amendment in 1971 that the voting age in the United States was lowered from 21 to 18. A big reason for this was the outcry that men were "old enough to fight, but too young to vote".

I would guess that the reason men were drafted into the military at age 18 was because of the need for as many soldiers as possible and since armor was no longer in use, a person did not have to be as physically developed to serve as a soldier. Additionally, most men finish high school at age 18 so they have all of their primary and secondary education completed.

  • Right, I know in US it was lowered from 21 to 18 in one of the constitutional amendments due to the Vietnam War. But the question remains why 18 is so popular in much of the rest of the world. – DrZ214 Jun 2 '16 at 2:42
  • I believe the reason for drafting men at 18 was to enable the completion of military service before men had married - as married men often had exemptions for obvious reasons - at least until the First World War. – Pieter Geerkens Aug 19 '16 at 5:22
  • I'm not sure that being an infantry private was or is less strenuous than being a medieval knight in armor. Many surviving suits of armor were made for boys who no doubt sometimes wore them. Infantrymen carry loads that might weigh as much as medieval armor and not be weight distributed as well. A man who watched Union troops march by during the US Civil War thought their 60 pound packs were too heavy, especially for the boys among them. – MAGolding Apr 29 '17 at 17:57
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On a minor detail, as far as I know ancient Roman law had several stages and ages of majority.

Roman boys had a ceremony to remove the toga praetexta with a broad purple border, and their protective bulla praetexa amulets, and assume the pure white toga virilis, the toga of manhood, at 14, considered to be the age of male puberty in ancient Rome. Roman boys would have their first military service sometime in their late teens. And I believe the end of adolescence and the age of complete adulthood was 25. And the minimum age to run for elected public office was 30.

So the closest fit between ancient Roman and modern legal ages is that the minimum age for military service is similar.

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    Do you have a source for assuming toga virilis at 14? Sources I looked at placed it normally closer to 17, only occasionally as early as 14. – justCal May 26 '16 at 22:57
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There were several reasons for 18 years old being the threshold of adulthood.

The first reason was military, as embodied in the 26th Amendment. During World War II, Congress lowered the draft age from 21 to 18, more or less in line with the other major combatants. This actually became a sore point during the Vietnam War, when the slogan became, "old enough to fight, old enough to vote." And "voting" was basically the key right of adulthood.

A second reason was that until the post war period, most children completed their (secondary) education at the age of 18, and entered the work force at that time. Here, again, they had "adult" responsibilities at age 18.

A third reason is that 18 is the maximum age of "consent" for marriage, and "marital relations" (including those undertaken outside of marriage). Although some states and countries have lower ages of consent.

It's true that for most people, particularly men, intellectual development is not complete, and perhaps one is not fully adult until age 25. (This is why a Congressman must be that age.) But that's not the point, which is that an 18 year old is "more adult than not," which is why the line is set there.

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