I think 15 is a bit of a high estimate. I read them in 5th grade and appreciated them even then. But then again, I read the whole Harry Potter series when I was 7, so I might not be the best example of normalcy. I think a fair amount people would understand them by 12 or 13 if they're mature.
Longmont, CO, USA
Amazing story with strong humanist elements. Read in conjunction with Narnia and be amazed at its answering calls to Lewis' fantasy and Christianity.
Cons: Not for children under 15 or so. And the third books gets a little esoteric and preachy. Also, don't read if you're Catholic - you'll just be offended.
A social location game - shows you where your friends are and rewards you with (meaningless) points and badges. Checkin everywhere you go. Become the mayor of your most frequented haunts. Pure Fun.
Cons: Sometimes people cheat and that's no fun for anyone.