"But over the last few decades, the age at which toddlers become diaper-free has been creeping upward. In 1957, 92 percent of children were toilet-trained by the age of 18 months, studies found. Today the figure for 2-year-olds is just 4 percent, according to a large-scale Philadelphia study. Only 60 percent of children have achieved mastery of the toilet by 36 months, the study found, and 2 percent remain untrained at the age of 4 years."
http://www.nytimes.com/1999/01/12/us/two-experts-do-battle-o...
The biggest difference that affects potty-training today is the time available for parents to dedicate to it, it has nothing to do with the children.
I think toddlers might have been a lot more motivated to learn using the potty when the alternative is sitting in wet cotton cloth.
Just so nobody without kids actually believes this:
It's perfectly possible to potty train some 1 year olds, lots of cultures do it, the hard part is communicating with them. Most 2 year olds can communicate verbally and by then it's mostly not a problem anymore.
Then, of course, they're all individuals and vary greatly even among siblings. But the idea that you shouldn't potty train kids before 3 is a strange and possibly damaging idea for new parents.
I don’t have any kids, but when I do I’m going to write an article entitled: How I helped potty train my two year old with a feather duster and an onion, or some such.
https://www.amazon.co.uk/gp/aw/d/B01JILAFG0
It fits in the rear footwell of the car for zero notice emergencies, has absorbent pads in the disposable bags and doubles up as a toilet seat adapter as well.