I found the easiest way to get younger players it to stress the "fun" of the game, rather then the "You must win at all costs"
I played 40k at a store with some youngish kids just getting into it, and there was one veteran who would just paste 'em with his Blood Angels.
I on the other hand, was trying to show them that the came could be fun, and wasn't just about beating the snot out of the other guy.
Tabletop face-to-face games offer stuff that Halo 2 really doesn't. Narative campaigns, joking across the table, and just generally relaxing and having fun without having to think about your "player rating" going down.
It's why I don't like GWs competative Campaign rules. They take something that should be fun, but they allow the uber-competative players to run roughshod over the guys who just want to sit around and laugh about a fictional Gretchin officer. With the exception of Blood Bowl, every other game encourages powergaming and rules bending.
If you start up a narative campaign, where losing doesn't mean you're down and out, you'll see younger players show up just because they see you having fun, and they want to have fun too.