When to start simple puzzles with babies?

Varies: I don't think there is an exact answer. The child would need to be able to manipulate the pieces. I think by 18 months, most children could do some easy shapes with help. Younger children might be able to do them and then some 2 year olds might have frustration with them. So try them and if they are too frustrating, then wait about 6 months and try again.