I think babies could be trained fairly early. I always had my kids on a sleeping/eating schedule from day one. Heck we potty train puppies why shouldn’t we be able to potty train an infant? I see more and more kids that are over 3 still in diapers.
I have known a kid almost 5 in diapers. The mom was just too lazy to teach the kids plus she wanted to keep him a baby. The kids granny told me a kid can be trained at 2 if you really try.
babies parents could be trained fairly early.
It's more likely that the parent learns each infants signals for elimination at that age . . .not the other way around.