Adult is a life stage in all games in The Sims series. In The Sims 2, it is normally the longest life stage in a Sim's life; in The Sims 3, it is the same length as the.
Young adult is an optional life stage introduced in The Sims 2 with the University expansion pack. In The Sims 3 and The Sims 4, it returns as the fifth life stage.
Okay, let's admit it there's a few of us (probably a LOT of us) who wouldn't mind our Sims' "whoo-hoo" being a bit more (*ahem*).