I think it is better to start when they have learned what is ok to swallow and what isn't. The early start can allow them to start building a new healthy habit that isn't hard and is vital to their hygiene. It will also allow the parents to teach the child about responsibility, self care, routine, and focus. A bedtime routine will help child to wind down before bedtime and fall asleep faster.
The sooner the better. Cavities develop easier on milk teeth than on adult teeth. And the sooner the child starts to floss the easier it gets for him to make it a daily habit.
When they can brush their own teeth (while still getting an extra brush afterwards by a parent). From around 6 years old they could have the motoric skills necessary. It would be good to start this early even if it is not necessary for hygiene yet, so they build the right habits for when they are older
It depends on what age the child start developing teeth. As soon as childhood starts, it's best to start when the teeth are fully grown.