I have 2 kids both boys, one almost 6 and one just over 2.
The oldest has played City of Heroes since he was 4 and now he has his own account and has had for 6 months, he loves playing both Heroes and Villains. We either team together or he solos. I tell him not to accept team invites cause although he can read he cant type well yet.
Interestingly playing City of Heroes helps him to learn to read and think about strategy.
Last weekend we had an 8 person team for the first time, he LOVED IT!!
Boy are we going to have trouble in a year when the 2 year old wants to play.