Am curious what is it that makes you fantasize over young girls as young as 14, and are they only girls whom you fantasize about? If that is the case then i must say, that is nothing else but lust. If you were fantasizing over boys as well then that would be a different case. You are at the age where you are trying to explore around, and tasting different temperatures of water. You better check what is triggering the whole thing before it gets out of hand. Ueually fantacies get people into trouble, you will find some either defiling young girls even in their own backyards, and that is very risky. Lust begins with what you perceive with your eyes then it goes to the mind before you know it, it takes over your well being and starts controlling the way you think. Be careful and do something about it.