I don't think they mind older kids going to a class for younger kids as much as kids who are too young for the class. I think there are safety considerations. It certainly couldn't hurt to call and ask, if your child is close to the stated age.
DS17(he was 16 at the time) went to a class for 8-15 year olds in August. They do ask for the child's age on the registration, so they knew he was "too old". Noone seemed to care. He was a little bored, but then so were the 12 and 15 year old in our group. I think 8-15 is too large an age range to keep everyone interested. Next year, I'm planning around a class for 12-17 year olds instead.
Marsha