Viagra is new enough that I can't answer this definitely yet, but I would guess that a 16-year-old who made a habit of it would join the ranks of the prematurely impotent sometime in his thirties or forties, just as cocaine users lose the ability to enjoy normal things without the presence of the drug--the beautiful world we live in becomes dull and gray for them.

it will probably work fine, however you shouldnt take prescription drugs without first conulting a doctor especially if you are young. the side effects will most likely be the same but you may be more at risk of getting them, again ask a real doctor.

Probably. The reason the anti-impotence effect of Viagra was found was that it was being safety-tested on university students when they were considering it for use as a heart drug. When they abandoned the test part way through, the male students were surprisingly unwilling to give back their extra supplies. It was only when they inquired about this that they found about its effects on erections.