There are a lot of confounding variables. Chief among them is someone at the top just wanting to get on with their life, start a family for instance, or basically anything other than study 12 hours a day.
It's hard to say it's cognitive decline for most of the people who just aren't working as hard at 40 as they were at 25.
If Chess960 or some other variant that doesn't involve as much rote work becomes sufficiently popular for long enough perhaps it will yield some valuable data about mental function versus age. At least a more holistic view than the studies we currently have.
