Hi K.,
My kids are 5,4,& 2-1/2. We have had them in swim lessons since they were about 9 months old. At that age it's more getting them used to the water than actual lessons. My two oldest attended a preschool that included a swim lesson, plus we keep them in a weekly lesson throughout the year. They are currently working on swimming without a noodle, but they can swim very well in a life jacket. My son is too young for formal lessons, but we take him in the pool every week and he can swim on his own with a life jacket and us right next to him.
The younger they are when you start, the better. I have found that kids at this age can move from being comfortable in the water, to having fear, then comfortable again. This is normal, but the older they are went they first start, the harder it is to break the fear.
We go to a YMCA for our lessons (this is also where the preschool is). The instructors are very good with the kids and do not make the child do anything they are not comfortable with, and I have yet to see any student remain fearful for more than a couple of weeks.
Hope this helps!
S.