DS happens to be very tall and rather athletic, so he will not be one of these tiny kids who seems young. He isn't young, in fact--he's a winter bday.
I have indeed heard that peer mentoring is great, etc etc. I should look up those studies. I tend to agree that it seems like it probably works when ability differentials are relatively small. I can't see how much DS is learning academically by saying, "No, this is an A." Socioemotionally, possibly, but it seems like there are some real Achilles heel possibilities.
