
I'm sure someone more experienced than I will be along to answer your query, but it does depend on the breed as to the best age.
Toilet training can begin as soon as you bring the puppy home, and in many cases a good breeder may already have started this... although the process of going to a new home can upset that, meaning you have to start again from scratch. However, if you anticipate starting from scratch anyway, then that's no loss.