How long is an average penis for a fifteen year old?

Penis size is much like the rest of physiology, you get what you get & what does it matter if you are bigger or smaller than average. The other issue is whether the measurement is flaccid or erect. There is a huge difference in the two from man to man. And there are no pills, potions, or creams that will make it bigger.
With that said, anywhere from 4-7" would be normal at 15 & most men are done growing by 20 so pretty much the message is "doing what you can; with what you got" is more important than worrying!
Napoleon, ruled much of the world & bedded Josephine, with a penis about the size of you little finger. So be proud of what you got & make love not war!