I'm 20, and the first time I was in a GW store is when I was 8 years old. I went with my friend's brother who was probably about 18 at the time. He told me the one golden rule. "Don't touch anything."
That's when I started to get interested in Warhammer. I remember that everyone was very polite to me, and they looked like they were having a lot of fun.
I think age has nothing to do with it. You're either have a good attitude, or you don't. I've had lots of fun teaching younger gamers how to play, while at the same time watching an adult's gloating and temper tantrums moving in and out like the tide.
One thing I always tell new players is to not take this game seriously. Gaming, fluff, painting, etc.
If you paint crappily, or don't, I don't care. If you're bad at the game, I don't care. If you don't know the fluff, I don't care. And if you want to play your hardest to win, I don't care.
Just do whatever it is you want without being a douche to me, and I'll play you.