Schala is right! I am not very tall, I have cup A breast size and extra small clothing size. I will never look like I'm 12 again , but I guess I can still pass for 16 or something (I am 19). And at my work, there was a girl who asked how old I was. I said "19". She said "Oh, thats young. I am 18". "You are younger than me!" I replied. She started to laugh, and said "It was a joke. How old do you think I really am?" I said "It's hard to tell, but I think you're 23, then." Guess how old she was? Thirty-eight!!
So you don't have to be ashamed of your fantasy, as long as you don't sexually assault people who are really that age. And if you stay ashamed of it, you can go to a therapist. He or she can probably helping you with finding out where this fetish comes from, because some fetishes like this have its origin in the youth (Not with all people I guess, I am also not an expert on this)