1. "My brain feels slow; I'm lost; must be that I'm not good enough. But I'm 18 and have to suck it up and try harder." Improves and masters self.
2. "My brain feels slow; I'm lost; must be that I'm getting old. Yes, that's it. Nothing to do." Lets go of themself; hello mental muscle loss.
3. "I bombed the exam. I'm just bad at [math, physics, foreign languages, writing, ...], but hey that's life!" Talks themself out of improvement and into a self-image that biases to failure.
4. "I bombed the exam. I'm over the hill so my brain isn't good enough. Oh well, but hey that's life!" Result: ditto.
5. "I bombed on the exam. I'm older, so I've forgotten how to try as hard as I used to. Time to get scrappy." Result: improvement and self-mastery.
Now add society. In the younger person a host of coaches, cheerleaders, friends and the dominant social narratives are telling urging them to push themselves further than they knew they could. "You can do it!" And with the old? "Hey, take it easy! Relax. We have low expectations for you--you can't do it--, and anyway we feel uncomfortable telling you what to do so we'll often just ignore you."
Without saying there's no such thing as age-related mental decline, I'm just about certain, seeing many older folks, that's it's abused, a self-fulfilling excuse, while in reality the truly age-related decline (as opposed to habit-related) is less important than it's made out to be, and can be deferred a decade or two beyond what many let themselves believe.
Similar to a person who can't give up the nice cars, house, clothes, vacations, dinners, in order to start all over again on a new adventure, too many older people have relied for too long on--grown addicted to--the luxuries of accumulated knowledge or seniority that allow them to operate at a broader, more conceptual level. Then, having to return to the pain of detail-oriented, working memory-related tasks feels as challenging as it did when they were youngsters. They were perhaps fresher, also typically more disheveled. Now they've lost the coaches and cheerleaders of youth and gained excuses--"I'm over the hill!"; "I require the dignity of age"--that cripple them. More exercise, healthy lifestyle, consistent effort, trying as hard at self-mastery as they did when they were young strivers and pushed themselves beyond their limits, and not listening to ageist arguments should make up for the lion's share of whatever mental decline is common in older people. They just need to be older strivers.
But the studies... Yes, well the studies record people who enjoy the flabby habits of seniority and the results of social bias. Far better to take control of the bias... to have the better, helpful bias.