6-year old son only wants to play with older kids?

I would be concerned that the school allows recess with the younger and older students mixed. It only takes a little sense to realize that a 7 year old is going to have vastly different topics of conversation than an 11 year old. I would restrict all extra curricular activities than involve being around the older kids and enroll him in activities with kids his own age. I would also allow the older sister to "tattle" in this situation. You can enlist her help on recess to tell him to go away or she will tell on him at home. In that way he knows someone is watching his behavior and actions. Also speak with the school and engage in a discussion on how to limit exposure of these older kids. That, it would seem, would be a huge concern for all parents. It certainly would be for me.