I think some teenagers and young adults are overwieght on purpose, just in case there is a draft. Plus I think that military conscription is weeding out the fit and leaving the physically unfit behind to reproduce. Think about this for a moment. Three consecutive generations of American men have been subjected to the draft. It may end up being the case that men who are 4-F are more likely to survive long enough to have kids.
I also hear that it is hard for a person with attention deficit disorder to get into the military. If its hard for an ADDer to volunteer, it's impossible to draft him. If we go to war with Iran, and have a draft as a result, we will become a nation of fat people with short attention spans.