I'm no expert, but
I don't think you can be 'gay' or 'bi' because of this. The important thing is to realize there's a difference between thoughts or feelings or urges and actions. Behaviors get you into trouble, not the stuff that goes on in your mind. As long as you keep them seperate and not act on them I think you're okay.
It sounds as if your attraction to younger guys has something to do with them being safer ('innocent', in your words), and that doesn't sound particularly sexual, either. Lots of things change and lots of things happen to us as teenagers, there are alot of stresses, and maybe there's something comforting ('attractive') to you about glancing backwards to a happier time.
(Of course, if you've been sexually abused, then all of this might mean something different, but since you didn;t say that, i'll assume it's something else).