You don't necessarily have to have measured the prevalence of myopia among children 60 years ago to extrapolate from the prevalence of myopia among 70yo today (since myopia is, by and large, a progressive condition and not typically reversed spontaneously).
Sure, you have to be worried about selection biases of all sorts including survivorship bias (eg. suppose myopic people mostly died before they reached 50), but in general, if myopia is as high or higher among 20yo than 60yo today, it tells you something pretty darn significant.
Sure, you have to be worried about selection biases of all sorts including survivorship bias (eg. suppose myopic people mostly died before they reached 50), but in general, if myopia is as high or higher among 20yo than 60yo today, it tells you something pretty darn significant.