Hi! Me again. DS is 8 going on 9. We went in Shark Reef a half a dozen times on our trip in May. No checks of age. I saw kids A LOT younger than DS going with their life jackets strapped on. I don't think there is any age limit enforced at all as long as the child can swim across without kicking his or her feet.
(We also went in Shark Reef last summer when DS was 7.)
I never saw anything about an age cutoff. I was there last week with my not quite 6 y/o. She didn't want to wear the mask and snorkel but wore her swim goggles to look at the fish. She and I just held hands as we swam through.