I did it with my 10 year old DD. There were a couple of other kids her age on with us, as well as a couple of college aged girls. The college girls were the only ones who fell off!
I do have to say that peer pressure is a funny thing. As we started, my DD was pretty excited, but the girl next to her was pretty scared. She was crying a bit. Her uncle was riding behind her and tried to encourage her, to no avail. Then my DD said "Isn't this FUN!!!" and she sniffed a little bit, smiled and said yes. From then on, she was prety upbeat about it.
DD got a little tired at the end, and was ready to quit!
Couple of pieces of advice. 1) Wear something over your eyes! The salt spray REALLY burns! I had on my prescription sunglasses with a strap on them. They helped, but I still got some water in my eyes. DD didn't have anything over her eyes, so she just squinted a bunch, and she did complain about her eyes burning. 2) Wear short or something covering your legs. DD and I got a pretty bad rug burn from the sides of the banana. I can't tell if the seats themselves had some non-stick stuff on them, or if it was just sand but your thighs will be happier if you have shorts on!
Overall, it was an absolute BLAST, and probably the best bang for the buck we spent on any activity.