In medicine and the social sciences, a young adult is generally a person in the years following adolescence, regardless of the local legal definition of "adult". Definitions and opinions on what qualifies as a young adult vary, with works such as Erik Erikson's stages of human development significantly influencing the definition of the term; generally, the term is often used to refer to adults in approximately the age range of 18 to 39 years, with some more inclusive definitions extending the definition into the early to mid 40s. The young adult stage in human development precedes middle adulthood.

In the literary business, the term young adult is very often used informally or in a marketing sense for the readers of young adult literature, books targeted at children down to ages 12 or 13. This broad extension of young adult to minors has been disputed, as they are not considered adults by the law or in most cultures, outside of religion (such as the Bar or Bat Mitzvah in Judaism), and the tradition of biological adulthood beginning at puberty has become archaic.