Different mental abilities peak at different times of life, from 18 to 70+ stroke considerations

Is your doctor taking into consideration these differences or just blindly stating you have cognitive problems?
http://digest.bps.org.uk/2015/03/different-mental-abilities-peak-at.html
This line is very important for your doctor to understand.
For instance, they suggest that tests for normal functioning in people who are suspected of having neurological illness or damage will need to be sensitive to the trajectories of performance through the lifespan for different mental abilities.