There may be as someone else has suggested a problem not with the childs age, but a problem with the child themself that you are unaware of. When my daughter changed from infant to junior school only 3 children from her year were going to the same school as her,2 from her class 1 from the other. I was very suprised when we went for the taster day at the new school to find that she had been put in a class with the child from the other class,someone she did not know, rather than the one she did.The next morning when i dropped her off at school her teacher took me to one side and said, i can`t say why, but i hope it will become clear very soon why i made the dessision to request they be placed in seperate classes, and asked me to trust her judgement. I did, and sure enough within a couple of weeks at the new school the other child was disrupting the class she was in and constantly blaming the children around her for the damage and trouble she herself was causing. Turned out she was quite an accomplished liar and a little light fingered with it,very sad as at that age it must of been learnt form home :-( but even so, even though i know its a selfish attitude i was glad that the teacher had acted as she did, at that age my daughter was quite shy and easily led, and i think the teacher had her best intrests at heart. Your may find there is a reason your sons teaching assistant is doing the same, normally like others have said, it would seem strange to stop them playing together just because of the class they are in.