Your problem seems to be with general knowledge of how the brainstem deteriorates as we age, every year. So of course his reactions were decreaing every year. the question is how much, which there is no way of knowing.
Actually there would be ................... but they would have had to have started him with a standard series of tests and continued those tests through today.
It is actually not unusual (and Mike would probably know of a few examples) where medical tests do occur across years if not decades to determine the standard set or level of deterioration of one's faculties.
It would actually (IMO only) be incredibly interesting to see such a series of tests done on high level athletes across their careers and into subsequent post career life (across a variety of sports as well)