Six years old does sound young, but there are so many (overnight) camps with extremely successful histories of creating a child's world for them. It's a judgment call as to when a child is ready. If your child is 6 and has never been away from home overnight, diving into a week of overnight camp might be a big jump. Families should create progressive experiences to get a child ready.
Parents are experts on their own kids, so they should trust their instincts. I've seen it both ways lots of families do bring kids and can learn a lot by watching their child interacting with camp directors, reacting to photos, slides. But if you have a very young child, it can be really overwhelming for them. It's a total judgment call.