It doesn't hurt to start them young.
A good starter camera is the Canon A410 (3 meg, 3X zoom). This camera can be found for $130.
Kodak has the C300 which sells for under $100.
It has no optical zoom.
Stay away from low priced no-name cameras.
These are often found on ebay.
They are junk and/or toy cameras.
Many people have had good luck with refurbished cameras. Make sure it comes with a 90 day warranty (or better).
I don't keep up with the used camera market, so I don't know what is available there.
My 9 y/o uses my Canon A70 with supervision and is quite careful and respectful of it. I would like to get him something for his own use. New/Refurb/used? Thinking simple, 3-4 megapix with Compact Flash and AA batteries. He already downloads pixs to his computer. Maybe a refurb or used Nikon 885/950, Canon A40? I really want to keep him interested. Any help on this will be appreciated.