Oh yeah, that was a biggie back in the day. I'm glad my parents were above thinking that way.

I think COD is a little much for a 10 year old. Just my 2¢.
I let my son play at 11 and I wish I'd held off another year or so. The problem is you can't control what other parents allow, and kids will do things at other houses they can't at yours, unfortunately, when their friends can. There's a difference between that and giving the kid something the parent doesn't agree with though.
It isn't just about violence, but language too. Too young and they don't ignore things as well as if you let them mature a bit.
My son is 14 and I still have his 360 locked down to friends-only comms in games. There are some idiots out there and if I heard him mimicking some of those taunting kids (and adults!) he'd be offline in a heartbeat.