my rule is if your not breaking it or killing youe sibling with it I wont say anything. They developed greed at about 2 and I am still working on the 10yo the 13yo gets it the 8 sort of gets it but the 4 is just like the 10 "Its mine" even if it originally belonged to someone else "Its mine now"
let me lookup some physc links it is early, practically from birth and has good reasoning from a specie survival viewpoint. The examples you gave are pretty typical of a white family though.
thanx,
bill