The best solution I’ve heard started by looking at who benefits from this norm [older women] and wondering whether they could have contributed to it.
While this is generally a good question to ask, at this point you would also need to think of a plausible mechanism by which older women could have contributed to the change. What new powers have older women (Would this be women over 30? those over 40?) gained compared to younger women, younger men and older men in this period that they could have used to change the norms so drastically? How would they have cooperated between themselves to thwart the other groups' desires?
A few centuries ago, we did not have the laws against child labour that we do today, and it was common for young children to work and support their families. This norm has changed, and I don't think we need to ask the question of who benefited and posit that there has been an increase in this-or-the-other group's power to explain the shift.
Nowadays in many parts of the world sixteen-year-old girls themselves have a say in who they hook up with, which is something a lot of societies in ancient times (and very many even now) did not grant them. This should definitely be a factor in your analysis!
While not exactly an answer to your question, this Economist article talks about certain situations where social norms and options available in the workplace etc might push younger women away from matrimony altogether, not just matrimony with much older men.
Why don’t men go for younger women? That’s not quite accurate, because they do, but we seem to have a cultural norm against girls who have recently exited puberty, even though they are highly fertile. Some time in the last few hundred years, we reached a cultural norm that as a man gets older, it’s increasingly less appropriate for him to be with a sixteen year old. That’s not long enough to override thousands of years of evolution, so what could have contributed to it?
The best solution I’ve heard started by looking at who benefits from this norm [older women] and wondering whether they could have contributed to it. After all, the strength of this norm has been increased in the last sixty years or so, which coincides with the period in which women’s power has increased.
One alternative I heard recently is that it doesn’t make sense biologically to go for women who are recently post-pubescent, as they don’t make the best mothers. Instead, a slightly older woman [mid-twenties] makes sense to be the best mother. This is perfectly plausible.
Another, less plausible, suggestion I’ve heard is that it’s to do with mental capacity. I find this unconvincing because we have few objections to a high-status man dating a beautiful but low-intellect woman.
Thoughts?