What proportion of the population had their eye sight measured 60 years ago in China (or anywhere)?
Is it certain that we are comparing like for like? Many more people everywhere do work that requires a lot of reading now than 60 years ago even in the northern Europe so perhaps myopia is more often diagnosed now simply because it is more of a problem in the modern environment.
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.
Is it certain that we are comparing like for like? Many more people everywhere do work that requires a lot of reading now than 60 years ago even in the northern Europe so perhaps myopia is more often diagnosed now simply because it is more of a problem in the modern environment.