Adulthood mixes biology, law, and coming-of-age rites
Biologically, an adult is an animal that has finished growing and can reproduce. Legally, a human adult has reached the age of majority and is treated as independent and responsible. Those two clocks rarely agree: a body may be mature at 15 or 16 while the law waits until 18, 20 or 21.
Majority ages range internationally from 15 to 21, with 18 the most common. Nigeria, Mali, the Democratic Republic of Congo and Cameroon set it at 15, though girls there are often married younger. In the United States you can vote or enlist at 18 but must wait until 21 for many legal and financial responsibilities. Reaching majority ends parents' legal rights and duties, yet it is separate from the minimum ages for contracts, marriage, driving, drinking, gambling and other activities, which may be set independently.
The categories often clash. Someone can be biologically adult and act maturely yet be treated as a child under the law, while a legal adult may show none of the responsibility associated with adult character. Historically and across cultures, puberty set the line, with signs such as menstruation, breast growth, facial hair, a deepening voice and pubic hair. People often passed straight from child to adult, marked by a test or ceremony, whereas most modern states simply name an age and ask for no proof of readiness.
Puberty usually starts between 10 and 12, around 10 or 11 for girls and 11 or 12 for boys, and ends by roughly 15 to 17, shaped by nutrition, genes and environment. During the Industrial Revolution children worked as soon as they were able, with little emphasis on schooling.
Claims since 2005 that the brain matures at 25 grew out of a Jay Giedd imaging study whose participants were at most 21. Newer research finds the prefrontal cortex still maturing well past 30, making the 25 figure outdated.
Source: Adult
