I'm not going to state this as a hard and fast rule, but somehow holding their breath and embolising seems to be an adult phenomena. We had an old bathtub down in a lake that was chained to the bottom (somehow) at about 10 feet, and filled with air. We used to snorkel down to it, pop our heads up, talk and what not, and then swim out, look around, go back, breathe, and then ascend. "Knowing" what I do today I'd never permit this sort of activity; but many, many, kids used the tub back then without any problems.
It reminds me of a story that Chuck Shilling (founder of the UHMS and Senior Medical Officer in the rescue of the submarine U.S.S. Squalus) told me, he always felt that embolism was a fear based problem and in a "natural" state organisms would simple exhale on their way to the surface. To test this idea he took a dog from the Harvard Dog Lab down in the "wandering bell" at the Submarine Escape Training Tower at the New London Submarine Base and tossed the dog out. The dog made it to the surface without any problem.