I started playing at the ripe ol' age of 19 and was fretting about the very same thing. Four years of heavy practicing later and I'm very happy with my progression and skill level. It doesn't make a difference when you start, I know some amazing guitarists who didn't pick up their axes until later in life and they totally rock, so be thankful that you're young and found something to be passionate about, but I don't think age has anything to do with how good you can become.