i agree with say12 with the 870, but dont get the youth model. get a express magnum the kid will do fine. i got a 20 gauge 870 when i was 10 and shot it with no problem i moved to full sized 12 gauges before 13. im not a big guy im 20 now 6' tall 160 lbs so im positive full size will be ok
ps... get a pump it teaches them to have to pay attention to the gun while they shoot intead of just keep pulling the trigger
"Let your gun, therefore, be the constant companion to your walks."
-- Thomas Jefferson