I think the hormonal issue is a good one to take into consideration. You'll read some prep logs, and hear about guys (or girls) seriously grinding it out, and just fighting their body tooth and nail to get that last bit of shed visible on game day. I'm certainly no doctor, but I can't imagine that it's a healthy situation, especially in the cases where people approach things in a less than intelligent manner and just crush their metabolism in the process.
As far as not gaining optimal size, I'll say this:
-If you don't have enough size now, then ask yourself if you think competing would be a worthwhile experience.
-Plenty of people compete in their teens and early 20's, and go on to pack on plenty of size. The argument that competing too soon would prevent later gains is overblown in my opinion. Yes, your body is in some prime growth years, and you would certainly be smart to capitalize on them, but, if it's something you want to do, then refer to my first point. What do you want out of the experience?
-If you haven't already, check out a local show and see the teenage class. In a 'tested' contest, people your age are going to look markedly different than those in an NPC (untested) show.