It's really not that hard. If they can kick their legs and hold their breath they learn quickly. I have mostly summer or early spring babies so they are in the water from a very very early age. 3-4 weeks depending on when their cords fall off. The hardest one to train was my 4 year old and he was 22 months but he was a slow learner at everything.
Wow! Never heard of someone swimming that early!
All ours except the 5 day old can swim. They can usually fully swim above and below the water by 15-18 months. We have pools and they are in the water from an early age.
My grandaughter was in the lake off our dock at 2. Learning to stand and hold the dock. And splash. Then the next summer, used a swim vest, then floaties, and then, suddenly swam across the family above ground pool without any help. The first lessons were about swim danger, and we found a wonderfull book about Stewie the Duck Learning to Swim. I think her parents will take her to lessons at the high school pool next summer. Years ago, we had a toddler drown just down lake from us, she was in the care of a 12 yr old cousin who lost track of her. It takes only a moment so we believe swim lessons are really important.