Young adult connotes younger people than "teen" for me. They use the term teen constantly in porn, and almost always for 18 or 19 year olds (or 32 years olds in pigtails pretending to be 18 or 19 year olds). Anyway, pretty standard by now. When you say young adult I think of "young adult" novels or other products aimed at the "young adult" market.... which usually means 14-16 year olds.