Answer:
There is no point where fitness is irrelevant to your age because if you stay fit your muscles will function longer and you will come across less problems with your body. I will maintain my level of fitness as I get order by doing the same workout every day and shorten it so my body wont give in the middle of when i reach that age.
Over time your body will break down and at which when it does you start to become more limited which is why you slowly regress the intensity of your workout with your new limits.