In my late 20's now and even my WRX is too much for me. Granted I'm learning to drive it better, but coming from low powered wagons, this 2.5 is beefy. I'm thankful my first car (that I got when I was 19) had only 163hp stock. It was still plenty zippy and fun. I can't imagine having a WRX or STi at 19; can't see any scenario where it's a wise choice though. Fun, absolutely. But certainly not wise. Maybe not even a good choice. =)
EDIT: I fully intend on doing a HPDE (or more than one =) ) to better learn to handle this vehicle. If I could afford it, I'd take real lessons at Sonoma or Laguna Seca but the NASA track days are far nearer my price range.