I'm 14 and I think that we have started playing the guitar earlier and more because we have got electrical appliances eg computers and TV at an earlier age than in the past and we are bored of them, so we need something new to do, like playing instrument (guitar being the best 
) or skating. Maybe we listen to our favourite bands/singers and think: I want to be like them/him/her. Also, you can get rid of anger by playing random chords really fast .
Next, music is good for people with head injuries or problems with their brain, a lot can play an instrument really well, but arent so good at school, but this isnt the case for all of them
Also, guitar hero is something new, it's fun to play with friends and win, then point and laugh like "ba-ha-ha-ha" It makes you better using your fingers for different strings/frets too.