One of the handy things about playing with other children is that one's peers are prone to providing immediate feedback on socially-unacceptable or unexpected behavior. One of the hazards of playing with much younger children is that sometimes they don't provide that feedback at the same stage (of behavior), or with the same clarity. It may be that he's not receiving their signals early enough to avoid hurt because they don't realize the potential for injury early enough, and he's relying on them to give him that feedback, rather than anticipating on their behalf.
