I started my DD1 at 6 months, but I feel classes for that age are a bit of a crock.
At 2, my dd1 started again. She's 5 now and can swim. She still does lessons and is up to stroke development.
My 3 year old DS didn't start until 3, but he was confident enough as we always took him in with us. He has been having lessons for 2 months and can already swim a few metres, but obviously needs a lot of work.
My 9 month old dd2 will start getting exposed to the pool and the beach now that the weather is warming up. She will start lessons at 3.
I'm not worried about her falling in as we don't have a pool.