In my experience, my mother wasn’t going to stand for, or wait for myself to be “ready” for potty training. It was the early 60’s, so it was cloth diapers, and plastic panties. Mom wanted done with the extra work ASAP! I was done with daytime diapers, (not by choice), by about 2 1/2, and around 3 at night. For the most part, most moms were aligned with this time frame. Most children were out of diapers by 3 1/2 latest.
I can remember my cousin (a distant one to me), who had a daughter in the mid 70’s. I couldn’t say how much the norm it was at the time, but her daughter wore disposable diapers up to 4 1/2. She wore diapers at bedtime till that old. My other cousins (1st cousins) who range in age, were mostly out of diapers, about the same as myself. Younger ones, who wore disposables, still about the same. My youngest 1st cousin, from the early 1980’s wore diapers till about 4. But that was at bedtime.
My nice, born in the new millennium, went to about 4. She sometimes asked for a diaper, even after potty training, but that only lasted a very short time. My mom, was not happy to see her granddaughter in diapers at 4 years, much less ask for one.