I'm not sure I totally agree with that. I am 26 and since I was 19 up until a year ago I thought I was a lesbian. But now for the last year I have been very much into guys. Growing up I never really thought about liking girls or wanting to be with girls (other than the occasional dream or whatever). But then I met a wonderful girl and I fell in love with her and we ended up in a relationship for 4 years. After that ended I met another wonderful girl and we were together a year and a half. When I was with them I was content to think I could spend the rest of my life with either one of them. Now, though, I am really into guys and while I am still attracted to girls, at the moment, I prefer to be with guys. I suppose that could change anytime now too.
What I am trying to say is that hitting a certain age does not mean you will automatically know how you feel about each sex. What I have come to realize is to not discriminate between girls or guys. I am content in knowing that should I come along a girl that I fall in love with and want to spend the rest of my life with that is fine. If I come along a guy I can't live without that is fine too. I search for love in all people instead of just guys or just girls.
I think right now you are in a curious stage. And thats perfectly fine. You have to try things out before you decide if it is something you like or not. Honestly I wouldn't worry too much over it. Just see how your feelings evolve for both guys and girls - don't limit yourself to one or the other unless that is what you find interests you. You are still a little young so don't rush into anything right now. Have fun, discover what you find attractive, what turns you on and if the opportunity presents itself to explore a relationship with either sex go for it - just do it in a responsible and safe manner. You have your whole life to discover who you are - at 14 don't rush to put yourself into one category or the other. And when you do find where you think you belong, don't feel like that is how you have to be for the rest of your life. You are always free to change your mind.