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.
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You can disagree all you want, but the new generations keep on popping out about every 21 years in most parts of the world.
For US blacks, it is closer to 16 years.
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