That's why he said 5. In America, that's when kindergarten starts. One pediatrician, Dr. Hodges, actually advocates potty learning at about 4, because he knows that part of what causes voiding disorders, and IC, is holding it too long.
Itsnoaccident.net
but then, in another study,
There's another method called Elimination Communication. The infants' faces and bodies, and cries are watched for signs of needing to go. The parent holds the baby over a potty place, and cues.
If a pottytunity is missed, it's the parent's fault, for not paying attention. The baby isn't punished, or rewarded, like one wouldn't punish failure while learning any other new skill. Kids at any age don't need a cookie every time they learn to do anything either. Diapers aren't out of the question with modern EC, either.
They don't seem to get that, F*%$k you! You lied to me! You let me believe this was okay for my whole life so far, and now, because
you decide, I'm supposed to magically know how to potty," thing typically trained kids have, and they tend not to be constipated.
The EC process is over around the time traditional training is started.
Keep things kind, gentle, and respectful, with no carrots or sticks, and there's merit in whatever you choose.
No kids yet, but, hey, forethought. Parenting is the most important act of philosophy anyone will ever have.