The authors suggest that if what you care about is when potty training is done, there is not much point to starting before twenty-seven months or so. But it is the case that after this, starting earlier does generally mean finishing earlier. If you start training at twenty-seven or twenty-eight months, you can expect to be done by around age three, but it will take ten months to do it. If you start at age three, you finish later, but it’ll likely take you less than six months to fully train.
