There is no simple answer to this as it involves a human dynamic. As we all get older the acuity of our senses becomes less and less sharp. Eyesight and hearing ability naturally reduce. Weight gain may become more of a problem. Blood pressure may rise. Memory becomes less sharp. Susceptibility to age related illnesses and diseases increases. All of this is part of the natural ageing process.
There are certain conditions that can not be safely tolerated in aircrew and they would certainly result in the withdrawal of a medical certificate. On the other hand many conditions can be treated and tolerated, sometimes with the application of additional safeguards such as "multi-crew only" etc.
Many conditions result in slow deterioration and they can be treated in conjunction with the persons own G.P's. Hypertension is a common example. A condition that be seen to be controlled to the reasonable satisfaction of the authority wouldn't usually result in the permanent withdrawal of a medical certificate.
Sometimes on this forum people with conditions beyond the initial issue limits laid down, fail to appreciate that those limits are not simply random pass or fail obstacles, but are limits that may reflect the fact that further natural age related deterioration may result in the early termination of their certification. Clearly this is unfair on the candidate and unfair on any prospective employer, both of whom might expect a fair period of medical certification if they are to have any confidence in the system.
Once issued, the authorities (certainly in the UK) go to great lengths to try and ensure that an individual does keep their certification for as long a period as is reasonably possible. This is often reflected in the fact that renewal limits are more relaxed than issue limits, and the discretion that is often applied to individual cases for the purpose of maintaining medical validity.
At the end of the day the importance is attached to treating the individual, not the medical certificate.