Don’t forget that the population is now aging, so we have many more people over the age of 40. In addition, awareness of visual disorders and testing techniques have improved, so more children are being diagnosed at an earlier age. And men now do make passes at girls who wear glasses, so it no longer carries a stigma.
Edited to add:
The norms are based on function, similar to the norms i use to measure hearing. Many people ask… “is my hearing normal for my age”? To which my reply is: “Your level of hearing impairment may be typical for people in your age range, but it is not ‘normal’... you are unable to hear many of the unvoiced consonant sounds, making it difficult to discriminate words at times.”
The same is true for vision… just because the majority of the population needs corrective lenses, so they are in the ‘norm’—but that doesn’t mean I want to drive on the highway with them not wearing corrective lenses!
Edited again to add:
Our youth are also reading and using visual devices (television, computers) at younger ages, and spending less time outdoors running around… this will change how the musculature of the eye develops… so I would venture to guess that proportionately speaking, more people are nearsighted than in the past.