Someone who has reached sexual maturity. (according to Wikipedia)

The age the law thinks that is varies from 13-18 all over the world. Are you sexually mature as soon as you are able to reproduce? Or do you have to have stopped growing? What about a 40 year old virgin? Are you an adult once you have kids? Are you an adult once you take responsibility?

Many 18-60 year old "adults" act like children, and many 15 year olds are mature also. There are 30 year olds that can pass for 18 year olds and 18 year olds that look 30. There are 30 year olds that mentally still 15, and 15 year olds that are wiser than 30 year olds.

There are typically three states of growth though... child > teenager or adolescent > adult Identifying where the second stage becomes the third stage varies massively from human to human. I think there is an argument to say that some humans don't become adults until much later in life