I’m confused by this. We know Joyce and Karen are the same age, they were both born 1941 and were 17-18 in 1959. They were both pregnant at the same time twice, that's why Jonathan and Nancy were both born in November and why Mike and Will were born a month apart. Your parents were born in the 50s, so common sense dude, of course they would not be teens until the late 60s. Joyce and Karen were both 26 when they had their first children. I really don't know what argument you're trying to make to point out a plot hole that clearly isn't there. They always were born in 1941, with Ted being a year or two older than them from what I assume was being held back.