I noticed I wasn't really into men at a pretty young age, around 13 or so, but I didn't realize I was looking at women sexually until I was around 17 or 18. I think I simply suppressed it because being bi was just a trendy thing, and I didn't want to be trendy. I also really wasn't aware of my sexuality for the most part. I just knew I wasn't really attracted to men, I thought women were pretty, and I had a pretty high sex drive, but no one I really wanted to enjoy it with. With all those things together it took me quite a long time to figure my shit out.
Fortunately I know better these days.