.
One generation is the average age at which a newly married couple begins their next generation.
In Isaac’s time it was 14 years, but now its more like 21 or so.
.
I disagree. I did not find that definition in either of my two print dictionaries, but I did find mine(more or less). Webster's New World #4: "The average period (c. 30 years) between human generations." American Heritage #5: The average time interval between the birth of parents and the birth of their offspring. Neither dictionary mentions the birth of the first child, or even the state of marriage. I am not challenging your personal belief, of course. The reason that I consider 33.3 years more real, is that in prior recent centuries, family size was much larger, with ten children being very common. I consider the generation in such a case to include all 10 children, not just the firstborn.