This post reminded me of another interesting thing I found in the Book of Ether which exemplifies its consistency. I read about criticism of the Book of Ether a while back where someone had calculated that there must have been an average age of 55-60 years between these generations in the royal line. According to them, this was "absurd".
If we assume that Jared lived around 2200 B.C. and the people of Zarahemla discovered Coriantumr around 590-440 B.C. the calculation is correct. I remember having read in the Book of Ether sometimes that they begat such and such at an old age. So I looked further into it. You could say that this is another case of an apparent "weakness" in the Book of Mormon turning into a strength.
We know it's common in most ancient cultures that the firstborn son becomes the next king. In Ether 6:25-27 when the first king is chosen, this is not the case. The next kings after that are born when their father was "in his old age". See Ether 7:3, 7:7, 7:26, 8:1, 9:14 and so forth. For half of the kings, it is specifically stated that they were born when the father was old. For the rest, nothing is said about age. It seems very reasonable to conclude that the Jaredites had a tradition where the youngest son became the next king. Perhaps that started with Jared's son.
Considering the fact that kings could marry much younger women and have children with them, 55-60 years seems a very realistic average age for having their lastborn. There were probably several who were younger, but we know from the text that there were also several who were older.
For instance, in Ether 9:23-25
23 And it came to pass that Coriantum did walk in the steps of his father, and did build many mighty cities, and did administer that which was good unto his people in all his days. And it came to pass that he had no children even until he was exceedingly old.24 And it came to pass that his wife died, being an hundred and two years old. And it came to pass that Coriantum took to wife, in his old age, a young maid, and begat sons and daughters; wherefore he lived until he was an hundred and forty and two years old.
25 And it came to pass that he begat Com, and Com reigned in his stead; and he reigned forty and nine years, and he begat Heth; and he also begat other sons and daughters.
Anyway, I find it interesting how you often can study something in some detail and it doesn't make sense or seems wrong. Then you study it in more detail and see how it makes sense after all when you have carefully considered all the available information. If anyone believes that Joseph Smith was making this up, at least give him credit for doing the math and making sense of it by leaving these hints of an unusual practice of lastborn kings without explicitly stating it.
I also have to say, at the time I studied this, I didn't realize that there were others who have studied and concluded the same, but apparently there is. So perhaps you have heard about this theory of lastborn kings before. I was made aware of the fact that there is a general term for this: ultimogeniture.