What is a good age to start a 16m for swimming classes?

Babies can be taught the basics of swimming with proper guidance but I think aged 3-4 would be the best time to start formal classes as they already have a better handle of certain movements like paddling and kicking themselves forward while submerged in the water. Make sure you and their teachers know proper CPR and don't force them if they're too tired. Proper breathing and stretching must be taught beforehand as well.