Echoing some of what Janet said,

1. Do NOT allow a young or weak swimmer to play in a pool unwatched, no matter how shallow. Not ever. Not even for a few minutes. Under certain circumstances, a child can become unconscious in only 15 seconds of submersion!

2. A child old enough to be in a pool without having an adult in immediate contact with them, is old enough to learn to swim.

My 25 year old son didn't realize pools HAD bottoms till he was 8 or 9 -- he'd always swum in deep public pools, and being able to touch the bottom was something he'd never experienced. At that point, though, he was swimming 2 - 4 miles daily in practice, and did not WANT a shallow bottom. He taught swim lessons for years, and prefers to teach children in water that is over their head.