Your issue here seems to be that you think nearsightedness/farsightedness naturally come from age. What happens often is that many people’s eyes tend to grow either too long or too short, which leads to near/farsightedness. This is not deterioration per sé; it is more like deformity. Your body simply grows the wrong way. The same goes for astigmatism and such. It is not caused by wear and tear. |||| Now, your eyes certainly *can* deteriorate. One example of this is that they become less able to focus in general; this means you get the worst of both near and farsightedness, and this is why bifocals exist. Another form of deterioration is cataracts, where the lenses of your eyes blur. The former comes from muscular strain, and the latter often comes from lens damage in your eye. These are often symmetrical and impact both eyes, as they *do* come from wear and tear. They are, however, usually not perfectly symmetrical as there is still plenty of randomness involved.
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