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.
The unprecedented amount of hand-wringing might not be indicative of an increase in the number or magnitude of sex-related problems if it turns out that previous norms also discouraged public discussions of such problems. What are the other metrics by which we can say that the current set of norms are working badly in practice? Are there fewer people having sex, are they having less enjoyable sex, or are their sexual relationships less fulfilling and of shorter duration or are these norms destabilising society in other ways?