The issue here is not age per se, it is consent.
The presumption is that most girls in their early teens have not yet mentally and emotionally developed to the point where they have the ability give informed consent to decisions that will have an enormous impact on the rest of their lives and the lives of their offspring. The justification for statutory rape laws, is to protect these children from older sexual predators until the young ladies have had a chance to mature further, and hopefully gain the skills necessary to care for themselves and their children if things do not work out.
A second problem concerns the special bonds which develop between juveniles (especially) and persons with special authority and trust in their lives: e.g. teachers, pastors, psychologists, physicians, etc. The presumption is that professionals who work with emotionally vulnerable individuals can use their superior skills and position to exert undue influence on the outcome of an encounter.
IRRC, according to the LDS historian and chronicler of Joseph Smith's wives, Todd Compton, Joseph had two especially successful approaches. To older woman, he would essentially ask, "Do you want to please your prophet or spend eternity in hell?" To younger women (i.e. teens), he would essentially ask, "It has been revealed to me that your prophet will be called home if he doesn't have sexual intercourse with you. Would you like to have that on your conscience? What would your friends and parents think about you if you killed their prophet?" In this case, given Joseph's position as a self-proclaimed prophet who was widely accepted as such by his flock, there is a serious question whether any of his numerous conquests were ever capable of giving informed consent.
In other words, we need to ask if Joseph was merely a pedophile, or was really he a pedophile as well as a serial rapist?
Smith also used the approach of assuring the girl's family members -- at least the parents -- a place in the highest degree of glory. It was essentially a form of "indulgences-prostitution" -- with Smith as the active sex partner-pimp. The crass offer, broken down, was essentially "sell your body & soul to me and it will go well with thy parents for eternity." He would simply further "spiritualize" it by asking them to pray about his propositions.
To this day, Mormon leaders of all stripes have shown they have learned those immoral lessons well. The fundamentalist MORMONS do that by continuing to rape girls and force teens into polygamy. The mainstream MORMONS do that by encouraging their sheep to "pray about" abortions before deciding if they should be subjected to one.
Both were part of Joseph Smith's curricula he taught.
Mormons of all stripes have yet to learn that you don't "pray about" moral issues God has already covered in His Word...lest you make them into ongoing quicksand issues that never become solid ground that you can stand upon.