You can predict cognitive ageing about as well from an eye test as from a cognitive test. What links these two? One possibility is that all systems age at a similar rate: This is supported by the finding that you can also predict cognitive ageing from muscle strength, or bone density. The implications are slimmer than you might think – evolution should get select for parts to age at the same rate just to avoid one rate-limiting component failing the whole organism, and to avoid over investing in parts that never fail by death.
However, here I am wondering if the association of aging eyes, ears, and muscles plays a causal role in ageing the brain.
I wonder whether (propose as a hypothesis that) when input and output hardware like the lens of the eye, hair cells in the ear, or muscle cell decays, the neurons listening/talking too them get less input/response, with more noise. And this flows back into the brain, making circuits less reliably related to the state of the world. That might then trigger apoptosis and pruning of circuits that have become bad – not because the neurons decayed, but because they no-longer have an input worth processing.