Diminishing or zero returns
"... If it is quite clear that you are winning most of the tactical slugfests that occur in your games, you may wish to make tactical play the central aspect of your game. ... If, on the other hand, no further progress is being made after 6-12 months of tactical training, it is likely that you are at a point of diminishing or zero returns. If you are still losing games through tactical mistakes, try to dampen the tactical potential of your games ..." - GM Nigel Davies (2010)
There was a study that showed as we get older the part of our memory that holds tactics starts to decline, where the part of the brain that holds strategy pretty much stays the same as we age.
Desire, is self explanatory. If you find yourself having to study more just to maintain? You might lose the desire to keep up the studying.
It was mentioned in a book by Boris Gulko: Lessons With a Grandmaster". From what i remember (im 55) i believe it was tactics, or pattern recognition that suffers as we age.