Our youngest son is 47 inches, so we spent quite a lot of time there in March! I agree with Wood Nymph - I didn't see strict enforcement. On the other hand, all the guests there were playing very nicely so I'm guessing they might enforce the rule more when it's either crowded or otherwise unsafe for littles.
At GF, we also saw older kids playing on the splash pad - all very nicely and it wasn't crowded. I remember a poster mentioning an older child turned away at the GF splash pad when it wasn't crowded. So it may vary by CM.