DD got a front door key wrapped up in a pretty box for her 12th birthday! It was quite literally the key to the door!
She didn't need it before then, because I was always in. Still am mostly, but it does save me from having to get up to answer the front door every half an hour, when she's been round the park with her mates.
Cant see anything wrong with 10 though. As long as he doesn't tie his name and address to it, it shouldn't cause a problem if it gets lost, should it?
16 years ago my mum gave us house keys - I was 12 and my brother was 10 (he lost his repeatedly but don't know if that was his age or just him ) we had key tags on ours like this one so hsi keys always found their way home without having to put an address on the keys (which would have been dumb obviously!)