I started with 13, at that time fist tube sites were available. And i can remember quite vividly, that there was always this one category, which seemed weird, but somehow also interesting to me. One day i clicked on it, maybe with the age of 14 or 15 and it got only down hill from there. So yeah, going against your generic sexuality at around age 15 is probably not a good idea...
The last spontanious erection i can remember was around the age of 16.