Start 'em young, I say.

I should set her up again, now that she's a bit older. She'd rather play Facebook Sims at the moment though.

I think Toontown online is a really good first MMO for kids.
It has restricted chat phrases. Open chat is available if code trading is done.

http://toontown.go.com/

I solo when I don't have hours to dedicate to a group. I feel shitty getting into a group when I know I'm not going to be there longer than 2 or 3 hours.