Historical fiction and historical fantasy are quite popular in some circles. (This includes fantasies based off of "historical" myths such as Robin Hood, King Arthur, etc).
Also, a lot of second world fantasy has a Medieval or "past" feel to it, including Tolkein itself. People read historical fiction or fantasy for all kinds of reasons - escapism, to learn more about the past in a fun way, a fascination with other cultures.
Which leads to modern sensibilities. In Game of Thrones, Daenerys is married at 13 (and in the books the sex is quite explicit - to the point where they had to make her older in the TV show). In Mercedes Lackey's Arrows of the Queen, Talia runs away to avoid marriage at the same age. At the same time I've been told when writing online that people were iffy with a character running away to avoid marriage at 16.
And here's the rub of it. For most of human history, marriageable age was "as soon as you're physically ready to bear children." Extended adolescence is a modern ideal - and while I don't believe writers should be going around promoting teen pregnancy or statutory rape, if we are writing in and about a society where such things happen, we shouldn't be afraid to have them happen. (Visual depictions are obviously a different thing, given the very strict laws on the matter).
At the same time, we shouldn't be afraid to write about non-consensual situations if they properly further the plot - and bear in mind that in these kinds of societies, arranged marriages were very common.
We can't apply the rules of our society to other societies - and speculative fiction writers should think very carefully before trying.