An adult is a person who has attained the age of majority. The age of The age of majority is defined by state laws, which vary by state, but is 18 in most states.
It's when you enter official adulthood for the first time, gaining great privileges along with serious legal implications. At the age of 18 you are legally considered an adult in nearly every state in the union. An 18-year-old can also buy and sell real estate and stock, inherit.
Biologically, an adult is a human or other organism that has reached sexual maturity. In human fertile, or who evidence secondary sex characteristics) and social adults (people who are recognized by their culture or law as being adults).