My daughter was 6 when she did it on last year's EBTA. I *think* she was the second youngest who stayed on that trip. I do remember asking if she was too young and they told me she was fine.
She did ask to come back to our room at one stage (because she wanted to sleep, and they were still playing...when she hits the wall she really hits the wall), but I went and saw her and convinced her to stay, cos I knew she'd be upset in the morning if she didn't. She stayed and had a ball.
Honestly though, you also have to be happy with it. There were parents of 8 year olds on that cruise who wouldn't let their kids stay as they felt they were too young. My daughter had already been having sleepovers at grandparents' houses, so we knew that wouldn't be an issue for her.