Biologically, an adult is a human or other organism that has reached sexual maturity. In human context, the term adult additionally has meanings associated with social and legal concepts. In contrast to a "minor", a legal adult is a person who has attained the age of A group of adult people. Historically and cross-culturally, Young adult (psychology) · Adult development · Adult (disambiguation) · Adultism. Definitions: Adult population by age group. Data presented for through are Vintage population estimates. Each year the U.S. Census Bureau.