My daughter was potty trained by 19 months. It wasn't a long process. It just happened over a day or so.
I do believe there is a window that most parents miss. When you wait past that window, there is more training involved.
But, some kids just really never get to that little window. I doubt it happens for everyone.
Kids potty train so much later now because the diapers are so nice. They are thin, and comfey, and easy. Kids have ALWAYS just gone in a diaper. Their whole lives. And, since it's such a convenient little system, why change???
My dd potty trained early because she wore pre-folds, and the disposables weren't all that thin yet. They were plastic, and had tapes instead of velcro. SHe also hated clothes. So, for her, it was easier to potty train than to have to wear cloth diapers.
And technically, I didn't train her. She kinda taught herself. (watching the dogs)