My brother bought the game when he was 16; I was 10.
If I didn't have my brother playing beside me and helping me, my parents had a very close friend who played the game too (He was 35), so I'd play with him online, and he would talk for me on teams etc.
CoH was extremely kid friendly. It actually kept a lot of kids (including me) away from more graphic games (Grand Theft Auto anyone?). I know games like Call of Duty and Halo have M ratings, but if you go on xbox live, you see much, much younger. If someone is 12 and all their friends are talking about how cool the game is, the kid is going to get his hands on a copy and try to play it, or play it at one of his friend's house a lot.
Someone mentioned ERP as a problem. I hadn't even heard of it in CoH until I was 16. ERP was mostly contained to Pocket D and other RP spots. Young kids don't really care to socialize, they want to defeat bad guys and run around the city. I never went to Pocket D except to see the light show.
Lastly, I really don't think someone who is trying to find out who they are (Anyone who is ~15), is prepared to roleplay a fictional character and get into that crowd. Does anybody remember someone under 20 RPing? I certainly don't. (hint:They all PvPed)