I've seen documentaries on families similar to the Browns and yes, the couples are young but not shockingly so. My daughter & I were watching a "Say Yes to the Dress" marathon on TLC and many, many of the brides-to-be were under 25.
sweetpea - I didn't read that book but I did read one about the Bountiful community. They are FLDS and they certainly do use polygamy as a weapon to wield control and power over the women of their faith. The church elders do marry girls that are far too young to be married and have been known to excommunicate young men in order to have more young women available for themselves. I'm very leery of grouping the Browns in with that kind of FLDS community because I think it's like grouping myself, a Catholic, with abortion-clinic bomber extremists when my beliefs are the exact opposite of theirs. I want to be sensitive to the differences that I think are the Brown's motivation for doing the show, but I think they would be better served by striving to explain the theological differences a bit more so we can understand fully understand them.