FWIW Manti in 1860 probably had a population of a few hundred, so the population of females aged 16-25 (or whatever upper age you think is reasonable) could have been quite small.

If only 2% of women over age 16 as also under age 18, that statistic is not so shocking. I don't agree that this statistic alone demonstrates that "almost all women were married between 14-16."

"A study in scarlet" is not historically accurate. IIRC, Conan Doyle said so himself.