I agree with Atatu.
The #1 guy in ITF for 2003, MArcos Baghdatis, last year beat Andreas Vinciguerra and Guillermo Canas in a challenger event and was top 200 by year's end. And of course you have the exceptional junior's like Nadal and Gasquet who are less than 18 and top 100 already.
It is a bit tedious but if you are interested look at either the ITF website or steve G's site and in the summer futures events you can usually see how the top juniors stack up against college guys. The juniors get a lot of wins.
I think one big thing like Atatu said is that top junior's don't necessarily go to school and basically play all day. NCAA requirements set some limits on practice time, schoolwork etc.
OF course the very best NCAA guys usually go pro after just 1 or 2 years of college.