Chess playing strength, like IQ, is a multifaceted thing. To my best knowledge, it's at least commonly agreed that when aging, you rather lose tactical attentiveness than positional understanding. I now could go endlessly into other facet details (Müller/Engel player types being influenced differently; risk avoidance; time management etc.) but since I shall only ask one question: Do you know of psychological studies dealing with the influence of aging on chess? (Generic aging/mental ability studies would be OK too if directly applicable to chess.)

Obviously, this is relevant for training.