In Sims 2, young adult and teen are separate life stages. Teens go to school from 9 am to 1 pm (0900 to 1300). Teens who go to college become young adults when they do.
Young Adult Sims only exist in college sub-neighborhoods. They're college students, and have one class per three-day semester.
Young Adults in The Sims 3 are not at University but are treated like adults. They can attend classes for various skills for several hours at the cost of §400.