No, no. It most definitely does not slow you down!
And it does not train you for puberty either.
I feel that masturbation does kind of train you for being prepared for having sex. Especially for the first time. You discover yourself and what makes you feel good. When you feel you're ready to hit the sheets with that some one, you know how you're body will kind of react. It has nothing to do with your actual body growth because in reality, everyone has touched themselves for pleasure since we were babies! My 3 yr old brother is like "whats that??" when he goes potty (lol). Ofcourse, we tell them to stop when they're little to avoid all those complicated explanations...Then you have us...we have more questions but are usually at that age where it's hard to talk to parents about it...
So I understand, there too! Hope that answered you're question!