Dont do it. If shes that young shes probably not had much experience in life. So heres your life if you do:
You'll have fun for a while, and then you'll have sex. She will think that having sex with you means you love her. She will start creating a much more serious relationship in her head than it really is. When you get tired of her being clingy and calling you 1000 times a day-you will try and break up with her. When you do she will go psycho and pull out every stop to get you to stay which will include:
-Im pregnant
-You knew I was underage and you still fucked me...well now if you dont get back with me you raped me
-If I cant have you i will turn into a psychotic bitch and stalk you
-I will stalk and possibly threaten every girl you date after me because you're "the one" and we should be together....
You get the point...run Forrest, run....