1) Probably a combination. My anecdotal observation is that there are more upper 30’s single women than men, so if you’re referring to shidduchim from 15 – 20 years ago, I lean towards saying that the main “difference” is the problem being noticed. I have been aware of the age gap cause, IIRC, for at least 12 years (before I ever heard of NASI). As an aside, my recollection matches AZ’s that shidduchim were first suggested to the boys in those days as well.
2) I would scratch my head and wonder if HKB’H had made a nes, perhaps in the form of a higher male birth rate than in the general population.