Please see #21. 20-25 years is too short. That may be the typical time when a person has their first child, but it is far from the time of the last child. Many people (especially men, and this particular subject is about patrilineal descent) have kids in their late 30s and even in their 40s - and this has been the case going back thousands of years. Average the age of the father when his youngest and his oldest child are born, and THAT will be the length of an average generation. Likely, the average is somewhere around 30 - 33 1/3 years.
This makes sense. Thanks.