And a Google search with words like "disposable diapers" "American" "late potty training" will turn up lots more information along this vein.
People seem to think that training children early is cruel. But there are gentle techniques for teaching continence, and we need only be in tune with the signals children give. The later age of toilet training indisputably correlates to the introduction and widespread use of disposables in this country. Did you know that Dr. T. Berry Brazelton signed on as a spokesman with Proctor and Gamble back in the 1960's? He was paid a lot of money to promote disposables.
Disposables are a big business in this country. Advertising and time have erased the memory of a time when most American children were trained by age 2. It can be done. It takes a little patience, knowledge of gentle techniques, and lots of praise. Children do tell you when they are ready to train, but these days people don't know the the nonverbal cues that children give us. Instead, we wait for children to stand up, walk over and say, "I'm quite finished with using diapers from this point forward."
The tide is slowly turning again, however. Perhaps your grandchildren will be toilet trained at 18 months as more and more people discover that it's far easier to train a younger child than it is an older one. The older child has basically unlearned an unconscious thing. Most people will accept that it's easier to learn a new language when we're younger rather than older. It's exactly the same for toileting.