The other day, I looked at Hall of Fame precedent and young hitters. The article is here, but long story short, you can already say with over 60% certainty* that Ryan Zimmerman will be a Hall of Famer (but go check it out for the reasoning).
*Not accounting for the BBWAA’s bias against third basemen, of course.
I figured,then, why not try it with the pitchers? I again looked at Baseball-Reference’s Play Index for Hall of Fame pitchers from the ages of 20 to 27 (from 1901 on), then looked at how today’s aces compared.