At a neighbors party a 2.5yo fell into the pool. I saw it happening and the little girl just SANK LIKE A STONE. With a panicked look on her face. Little feet hit the bottom, let out a blurp of air and did nothing. Her Dad was already in the air diving into the water before I could get up. The child was fine, thankfully. Her folks immediately asked about the classes I'd mentioned before.
I can still see the complete bewilderment on that child's face. She just had NO IDEA what to do and would have drowned had it not been for quick actions.
My point here is lessons are absolutely worth considering starting MUCH earlier than you might guess. This isn't learning to 'swim', it's learning how 'not to drown'.