Emerging adulthood has been recognized as a concept in sociology for over a decade, and is generally recognized as extending to age 30. Adolescent health research often extends up to age 25 because brain development continues until about that age. Last year I asked Alan Guttmacher, the head of the National Institute for Child Health and Development (NICHD), about the definition of adolescents for the purposes of NICHD funding, and he said that it's reasonable for research to go past age 18.
In view of that, I'm submitting a new manuscript today, and as I was choosing keywords, I was surprised to notice how they classify ages.
Adults are ages 19-44. Young adults, a term added in 2009, are ages 19-24.
My study is about the transition to adulthood --- how people get college degrees in early 20s and develop professionally in their late 20s and early 30s --- and it doesn't seem to fit into either of these categories. The study examines how the subjects are in the process of becoming adults, socially and economically, but according to the medical subject headers, they are already. It's striking to be reminded of how our systems of thought have changed in such a short time, and an indexing system naturally didn't catch up.