|MadSci Network: Development|
Of course you checked the dictionary and found something like the following:
adult [L. adultus grown up] 1. having attained full growth or maturity. 2. a living organism that has attained full growth or maturity.
There is no age, however, that objectively defines human beings as adults. It depends on when an individual completes puberty. Puberty is a "complex biologic and psychologic process involving sexual development, accelerated growth, and adrenal maturation heralded by the secretion of gonadotropin releasing hormone (GnRH) from a part of the brain called the hypothalamus. This event is the first known step in the reproductive cascade. It initiates the pulsatile release of gonadotropins, gonadal secretion of sex steroids, pubertal development, and gametogenesis (the production of sperm and ova)." - except from http://www.medterms.com/script/main/art.asp?articlekey=5118
It could be argued that a human being is an adult when they reach sexual maturity and are capable of producing viable offspring. This is how we tend to think of adulthood in other animals, but people have differences in opinion and the semantics can be quite intense -- especially between parents and their "adult" (as defined by onset of the reproductive cascade) children!
Hope this helps,
Try the links in the MadSci Library for more information on Development.