Babies under 18 months of age can be trained to use the toilet and hold it in.
However that is elimination communication and not completely toilet trained.
If a baby is too young to walk properly to the toilet then it's not a good idea to have them go without wearing diapers.
I would say any baby 18 months or under is really too young to be potty trained and should be wearing diapers full time.
My sister tried elimination communication with my nephew and she wanted me to do that while I was watching him as well when he was 8 months old.
However it was not a good thing to do and I kept him in diapers.
Babies that are 18 months or older can be potty trained.
Most kids are not ready for potty training until 18 months to 2 years old and some kids are not ready to give up diapers and be potty trained until 3 or even 4 years old.
There's no need to force the potty training issue until the child is 4 years old and nearing 5.
Babies are only little for so long and so you should not rush potty training.
They are just too little to grasp the concept of using the toilet until around 18 months of age and so they should be wearing diapers until at least 18 months.