I think it is the course of a society that is integrated with technology that as people become lazier with their parenting and leave it more to the electronics than ever before, potty training will forever continue to come later and later. It was uncommon for kids back in the cloth diaper era to be left in them past the age of two, but nowadays not only is it common but damn near acceptable to have your kid still wetting and messing themselves near-uncontrollably at that age. Potty training age is loosely pitted now as "before school starts" which can be anywhere between three and five years of age depending on where you live.
Here in Ontario, the Kindergarten program is optional so kids don't technically have to start school until the year they turn six. If they go into that, they can enroll in the year that they turn four. So if your kid's birthday is December 31st, you're good to start enrollment procedures for September starting January 1st.
However, to go back to the six part. As kids spend more time on electronics, parents may find themselves replacing elementary teachers with electronic devices. Which, while not as effective in most cases, would work, but would also give the parent even less of a reason to toilet train until after far as six.
I also don't think we're at the point where we have enough data regarding this next thought, but I wonder if kids who are potty-trained in later years (3 through 5) would also end up training their kids at that point because that is their experience with the age of which it happens, assuming they have memories of the time at all.
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