Adult

• 1. Introduction

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  An adult, in the strictest biological sense, is simply an animal that has reached full growth. That definition sounds settled until you apply it to a human being. A person can be physically mature by age 16 yet wait years longer before the law agrees they are grown. In the United States, you cannot join the armed forces or vote until 18. You cannot take on many legal and financial responsibilities until 21. So which clock decides when a child becomes an adult? The body has one answer. The law has another. Culture, religion, and the human brain all keep their own time. Across the world the threshold ranges from age 15 to 21, and even within a single country the rules contradict one another. What follows is the question this raises. If someone can be biologically grown, behave like an adult, and still be treated as a child, then what exactly are we measuring when we call a person an adult?

• 2. The Body's Verdict

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  Menstruation, deeper voices, facial hair, the development of breasts and pubic hair. These secondary sex characteristics are how adulthood was determined for most of human history. The biological definition rests on a single capacity, reaching sexual maturity and becoming capable of reproduction. Many dictionaries phrase the first meaning of adult as the stage of the life cycle of an animal after reproductive capacity has been attained.

  Puberty usually begins between 10 and 12 years old, but it does not run to a fixed schedule. Girls typically start the process at age 10 or 11 and generally complete it between 15 and 17. Boys typically begin at age 11 or 12 and finish by age 16 or 17. Nutrition, genetics, and environment all shape when the onset arrives and how the changes unfold. Girls go through a growth spurt and gain weight across several areas of the body, while boys experience their own spurts in a different style and time frame.

  The Industrial Revolution treated this physical readiness as enough. Children went to work as soon as they could to help provide for their families, with little emphasis on school or education. A child could get a job without the experience demanded of adults today. In many societies a person moved directly from the status of child to the status of adult, the shift often marked by a coming-of-age test or ceremony.

• 3. The Twenty-Five Myth

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  Since 2005, a striking claim has circulated in the media, that the human brain does not finish maturing until age 25. The figure points to the prefrontal cortex, an area said to remain unfinished even at 18. The number sounds precise enough to set policy by. It is also wrong.

  Jay Giedd produced the brain imaging study at the root of this idea, dating back to 2004 or 2005. The study had a hidden limit. Its only participants were aged up to 21 years. Giedd assumed the maturing process would be complete by age 25, and that assumption hardened into a media fact. More recent studies tell a different story, showing prefrontal cortex maturation continuing well past the age of 30 years. The neat end-point of 25 turns out to be an interpretation that was both incorrect and outdated, which leaves the science of full mental maturity arguing over whether such an age exists at all.

• 4. The Patchwork Of Law

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  The age of majority is the moment parents lose their parental rights and responsibilities over a person. Internationally that moment ranges from age 15 to 21, with 18 the most common. Nigeria, Mali, the Democratic Republic of Congo, and Cameroon set it at 15. At the other end, Mississippi and Puerto Rico in the United States, along with Bahrain, hold the line at 21.

  Nineteen is the threshold across a swath of Canada, including British Columbia, Nova Scotia, Nunavut, and Yukon, and also in Nebraska, Alabama, and South Korea. Even within these jurisdictions the rules splinter. Canadians may be treated as legal adults at 16 for sexual consent and at 18 for criminal law, federal elections, and the military.

  Reaching the age of majority does not unlock everything at once. A jurisdiction may set separate minimum ages for signing a contract, marriage, voting, holding a job, military service, owning firearms, driving, traveling abroad, alcohol, smoking, and gambling. The law also distinguishes between commercially enabling a young person and socially enabling one. Sometimes it requires supervision by a legal guardian, sometimes only by an adult, and sometimes it offers nothing firmer than a recommendation.

• 5. Old Enough To Fight

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  Old enough to fight, old enough to vote. That slogan rewrote American adulthood. Before the 1970s, young people were not classed as adults until 21 in most western nations. In the United States, citizens could not vote in many elections until 21. Young men between 18 and 21, meanwhile, were being drafted to fight in the Vietnam War. In July 1971 the 26th Amendment passed, mandating that the right to vote cannot be abridged for anyone 18 or older.

  That single change pulled other ages down with it. Many states lowered the drinking age, most settling on a minimum of 18 or 19, and many lowered the age of legal majority to 18. The retreat did not last for alcohol. As of July 1984, the National Minimum Drinking Age Act required all states to raise their drinking ages to 21, a response to drunk driving deaths among young drivers. States that refuse to comply can lose up to 10 percent of their highway funding.

  Twenty-one survived as the benchmark in pockets of American life. The Gun Control Act of 1968 bars anyone under 21 from buying a handgun from a federally licensed dealer. The Credit Card Act of 2009 forces young adults between 18 and 20 to find a co-signer who is 21 or older, or to prove they can repay the balance. The federal government later raised the age to purchase tobacco and vaping products from 18 to 21. The Affordable Care Act of 2010 reached the other direction, letting young adults stay on a parent's health insurance up to age 26.

• 6. The Gray Zone

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  In March 2021, the Supreme Court of the State of Washington threw out the life without parole sentences of a 19-year-old and a 20-year-old in a 5-4 decision. The two had been convicted in separate cases of first-degree aggravated murder decades earlier. The justices ruled that, as with juveniles, a court must first weigh the age of those under 21 before sentencing them to die behind bars. The case landed amid debates over whether people between 18 and 20 should be exempt from the death penalty.

  Germany takes a parallel view inside its own courts, where defendants under 21 are largely sentenced under juvenile law. The aim is to help them reintegrate into society and to fit the punishment to both the crime and the offender. Other recent laws push the line the same way. In May 2021 Texas raised the age to work at or patronize sexually oriented businesses from 18 to 21. In Quebec, the legislature raised the purchase age for recreational marijuana from 18 to 21 in 2020, citing the risk to brain development, while most of Canada kept it at 19 and Alberta at 18.

  Not every rule moves upward. In the United States youth can take a part-time job at 14 with a work permit. At 16 a person may get a driver's license depending on the state and, in some states, consent to sexual activity. At 17 one can enlist in the armed forces with parental consent, though deployment into combat roles waits until 18. A handful of places run faster still. In four Maryland cities, Takoma Park, Riverdale, Greenbelt, and Hyattsville, 16 and 17-year-olds may vote, and in 2020 students 16 or older in Oakland, California won the right to vote in school board elections.

• 7. A Social Invention

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  Aging is biological, but adulthood, social scientists argue, is constructed. The proof is that its criteria keep changing. Historically in the United States, becoming an adult meant finishing one's education, moving away from the family of origin, and beginning a career, with marriage and parenthood close behind. These markers are subjective, organized by gender, race, ethnicity, and social class, so some populations feel adult earlier in life than others.

  Younger cohorts have traded those visible milestones for something more internal. Research finds a growing importance of individualistic criteria and the irrelevance of the demographic markers of normative conceptions of adulthood. Three things now define the threshold for many young people, a sense of responsibility, independent decision-making, and financial independence.

  Jeffrey Arnett, a psychologist and professor at Clark University in Massachusetts, gave this shift a name. He argues for a distinct period he calls emerging adulthood, occurring between the ages of 18 and 25. These individuals can take some responsibility for their lives but do not yet fully feel like adults. Arnett identifies five features unique to the stage, identity exploration, feeling in between, instability, self-focus, and having possibilities. The last he describes as an age of possibilities where optimism reigns, a time when young people believe they have a good chance of turning out better than their parents did.

• 8. When Faith Sets The Age

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  Age 13 for boys and 12 for girls. In Jewish tradition, the Bar or Bat Mitzvah marks adulthood at those ages, with the young person expected to demonstrate readiness by learning the Torah and other Jewish practices. The Christian Bible and Jewish scripture, by contrast, set no age requirement for adulthood or for marrying.

  The 1983 Code of Canon Law fixes its own thresholds, holding that a man cannot enter a valid marriage before completing his sixteenth year of age and a woman before completing her fourteenth. The history runs harsher still. In The Disappearance of Childhood, Neil Postman records that the Christian Church of the Middle Ages set the age of accountability at 7, the point at which a person could be tried and even executed as an adult.

  Religion does not simply mark adulthood. It moves with it. The National Library of Medicine points to studies showing religiosity declining as people move out of the house and live on their own, changing their life goals as they discover who they are. Other studies show the reverse, a rise in religiosity as adults marry and have children and settle down. Everyone's level builds at a different pace, which means that religion relative to adult development varies across cultures and time.