The polygamy part. But not necessarily the age part.

Was child marriage really normal back then? The stats I've seen say 19th Century women were getting married in their early 20s and men in their mid 20s. It went down by a few years in the 1950s and 1960s before going up to what it is now.

1890s/1900 was actually a local maximum in women's marriage age that would not be equalled until about 1980. http://users.hist.umn.edu/~ruggles/Articles/Fitch_and_Ruggle... shows 10% of native-born, white US women were married by age 17 in 1850 (first year of data), and that 10% threshold rose rapidly through the latter half of the century, so it may have been more common to have very young brides earlier in the century. Though at 10% even in 1850, it was not unusual. Add confounding factors like immigration and poverty, and young brides may have been even more common (and vulnerable to be protected or taken advantage of depending on your perspective)

> This is a common rationalization taught to educated true-believing mormons.

FWIW I'm not a Mormon (or an American). So I have no dog in this fight. I just find the historical demographics & statistics interesting.

> [...]age of marriage was about the same it is now (http://i.imgur.com/HnQjG67.png).

Yes, if you look at the average for the population as a whole. But once you look at percentile breakdowns (such as page 26 of [1]) you see a very different picture.

In 1850 the 10th & 25th percentile of marriage age for white women was 17.0 and 18.8, respectively, 16.2 and 17.8 if they were black.

It seems very likely that if you were a sustenance farmer on the frontier in a town that had been established just 10-15 years prior (such as Manti, UT being discussed here) it wouldn't be unusual for "normal" locally to be what was the "normal" for the 5th or percentile of the population as a whole, or even lower.

Furthermore as the [1] paper notes the marriage age (and childbearing) ages in Western Europe and North America were inflated over the rest of the globe at the time, due to a difference in societal structure.

I wouldn't be surprised if isolated communities in North America such as the Mormons or the Amish who were deliberately setting out to do their own thing looked very different demographically.

> Sally Hemmings and Thomas Jefferson (15 and 45 respectively) in the late 18th century and you can clearly find plenty of condemnation of the age disparity between the two.

Yes, that sort of thing is definitely an outlier, and the Mormons with their polygamy practices can rightly be said to be unusual in this regard.

But that shouldn't be confused with the question of whether the age of first marriage (or child) should per-se be considered profoundly unusual for the time, or some extreme outlier. As far I can tell it wasn't.

1. http://users.hist.umn.edu/~ruggles/Articles/Fitch_and_Ruggle...

2. https://news.ycombinator.com/item?id=19131894