All three of my kids started out in a week-long Y Camp, which was really only six nights. They all started around 7 or 8 years old and had generally good experiences, though there were some aspects of that particular camp's experience that I wasn't real fond of. Still, it was nearby, it was short, and it always felt like a good "training wheels" kind of program.
Now two of my kids have been to camps that last 3 weeks (starting at ages 13 and 15). I was really worried last summer, sending my 13yo to a 3-week-long camp where there's absolutely no electronic communication, especially since she didn't write once while she was gone. I was so worried that my quiet, introverted kid was miserable! But when camp ended and we went to pick her up at her cabin, we found her right in the middle of a massive dogpile of cabin-mates, happier than I have literally ever seen her. She adored it, as did my eldest child).
I will say, I am still really worried about the camp DS will be attending for 4 weeks this coming summer. I'm not at all convinced it's a good fit for his personality. But he really wants to try it, and I figure I have to let him try. If all else fails, he'll learn how to deal with an imperfect situation for a few weeks.
DC1 -- 2005 DD -- 2009 DS -- 2011