For anecdotal evidence you can look at the age polls on Minecraft forums which show 60% are under 21.

Don't forget that reddit isn't particularly child friendly, and yet even their the MC subreddit is younger than most of the rest of reddit.

Wow, I just ventured in there for the first time to confirm or deny your statement, and you are totally right. It is packed with kids. I guess minecraft is the new lego.