I don't know about Centreparcs, but in other places I've found that a minimum age is strictly enforced (probably insurance or supervision ratios) but there is sometimes flexibility for a (slightly) older child to stay with a younger sibling in something that has a maximum age (provided there is space after all appropriately aged children have had a chance to book).
I'm not sure if your concern is gentle activities for younger children being invaded by bigger ones, or if you have a younger one desperate to try something reserved for older ones. But either way, it will end up more or less exactly what is advertised.
Thanks all for the advice. DS wanted to do "Family Foil Fencing" with his dad but would need to be 9 and he's a couple of months out. I wondered if it might be OK as he would obviously be under his dad's supervision but it sounds like a not!