Yeah, Supergirl was nearly always a teenager. She's only just been aged back up to 21, which is roughly how old she was pre-Crisis. She was never old enough to be a mentor, I think Supergirl: Woman of Tomorrow was the first time she was shown to be one (to Ruthye). Power Girl though, is older, and is a second generation JSAer (along with the rest of the Infinity Inc generation). So she can serve as a mentor to the third generation (Stargirl's generation), and other teen or preteen heroes, such as Jai West.