I think everyone is doing them at Lifetime Fitness around here. The starting age is 9 months. Others have suggested the Y also, not sure of the age on that one though.
Thanks for the Natatorium rec., FYI for anyone thinking of them: I just called them. I got on the wait list for June (6th -27th) classes. They said they have 4 names on the wait list now and and if they get a few more for May they will probably open another class. If they don't, I'll be doing the June sessions instead. I think they had one April class open, but we can't make that.
Re: Those who have done/are doing/ going to do swim....