Holy Shit! Don't do it!!!!!! YOUR PENIS WILL A'SPLODE!!!!!!
I'm NOT kidding. Well.... maybe exaggerating...a little. But think about it: at your age enough blood already enters the blood vessels of your penis to get a normal erection. If viagara increases the amount of blood TOO MUCH blood could enter these vessels and cause them to rupture! (A'SPLODE!). If that happened, you would have NO healthy blood vessels in your penis anymore, and without major reconstructive surgery (which may not even work) you could NEVER have an erection of any real strength again!!!!
A'SPLODE! (sorry I like that word....).