Well I read it in Men's Health but I can't find the article so I can't tell you where they got their info from, HOWEVER, I had a friend who started taking viagra when he was 17-18 (now 22) for no reason and little by little he built a tolerance to it. Today he's up to 200mg now and it is hard for him to get an erection without it.
I know that's only one case but it's pretty damn convincing to me.