My belief had been that when they are dry thru the night, they are ready to go potty. I think that just like walking, talking, etc., the control is developed as they grow. Some sooner than others.
My psycology professor said, if the were trained really early that it was the parents trained not the child.
Except for my first child who was trained around two, the others trained themselves, my daughter by 18 months and my second son I don't remember but I know it was before 2. They never had accidents. If a child is not reaady it is too much stress on both the parent and the child.