Many people lose the mental capabilities they used to have when they were young, so they become unable to act the same way as they used to. They often lose the ability to learn new things. We’re talking 60+, so there are hormonal changes that can influence both the physical and psychological side of a person. Just stay young in your head is an advise similar to saying “cheer up” to a depressed person. The mental decline is probably partly genetic, partly pure luck or lack thereof, and a minor part can be influenced by lifestyle. So I guess try to influence what you can, excercise, eat healthy food, read, meet younger people and hope for the best.