Hi again mominsac,
My son went to cty stanford as a 6th grader (but only age 10). I really liked how all the kids were basically the same age group. He had a great experience, both with the class and socially. It's actually surprising to me that he got along well with his roommate because he can be grumpy. But his roommate was from Shanghai and was really nice (and I'm sure tolerant). It depends on the child--I would definitely consider sending my next ds as a 5th grader (age 9) if he's interested because he's is more organized that his older brother and more sociable.
I had sent my 11 yr old previously to a one wk program at univ of denver (through the formerly known as Rocky Mountain Talent Search, now known as Western Academic Talent Search) and to other one-week adventure-type camps. These were "warms-ups" for a longer (3-wk) stay. I don't know if I would send my son to a 3-wk program as his 1st sleep-away experience, and would recommend some kind of "warm-up" if you haven't already.