Depends on goals
To begin with, no one at any age can be drown proofed.
Learning to swim well is often something kids> 4 yr can retain & enjoy. Under that age they have difficulty comprehending water danger & may venture off & swim unsupervised, leading to tragedy.Over 4 your warnings can be effective. Babies can be introduced to the water & helped to enjoy it, but i frown on any baby swim classes.