First children didn't usually come when parents were older, but when people had lots of children women (and of course therefore men) were having children well into their 40's. When there was no reliable birth control, basically menopause was the cutoff point. So those later in life babies had older parents.
Growing up in the 50's I had lots of friends who were the "second batch" or "oops" baby in the family, just because of this. You are looking at a very short period of time in history, when people can generally control how many babies they have and when. The old method of birth control was marry later in life (i.e. 30's, not teens or twenties). And people still "had" to get married, because pre-marital sex still happened.