You're expecting FOs to fly with Captains close to retirement, who they suspect might be declining mentally, to FSAP this or have documented refusals to fly with them? I'm sorry, but this is incredibly naive. What should the FSAP say? "I just finished a single pairing with Bob and I, the First Officer, hereby diagnose him with Alzheimer's." If the same FO gets paired with Bob again, do you honestly think he's going to march into the CPO, diagnose Bob with dementia on the spot, and refuse to fly with him? Or, do you think the FO might just call in sick/trade and add Bob to his avoid list?

Our industry is not set up structurally to catch the data you have described via safety reports, etc. In other words, you're pointing to the absence of data that would never exist to pretend that known age-related cognitive decline doesn't exist.