Age of 18 is the standard U.S. age for graduating from high school and going to college which, in turn, means the typical age that most youths live on their own for the first time. Also, it’s a common age in the U.S. for age of consent. Living on one’s own and legally consensual sex are two milestones in reaching adulthood.
However, according to U.S. tax laws, a person isn’t fully adult until age 25. One can be claimed as a dependent child between ages 18-24 if a full-time student or disabled.