I tend to agree with you - 8 is probably the youngest I would want my kids to be on these trips. You want to be able to enjoy yourself, and let the kids have a bit of independence hanging out with the other kids and making friends, which is easier when they are a little older. We took my girls in 2008 when they were 9 and 10, and it was perfect (and again this year, when they were 10 and 11, also perfect). These trips are too expensive to have to spend them catering to a grumpy kid - you can do that at home!
It may also depend on the trip and your kids' personalities. If they like meeting new people, seeing new places, and don't mind being on a schedule that they have no control over, they will enjoy a group trip! If they hate bus rides and don't have a lot of patience for waiting, you might want to reconsider (depending, again, on the trip).