Elderly grow just as many brain cells as youths:
You’ve probably heard the factoid that only children grow new brain cells. It was never true, nor could it be true. In fact, as we age, there are increased neural network connections which allow certain skills to be acquired faster and certain problem-solving to be done more acutely. This is a well-known phenomenon in the arts, business, and military strategy, which has been increasingly bolstered by scientific findings in the past fifteen years.
We must differentiate what can be possible from what happens on average. The average people make no effort to preserve health, to learn new skills, to dramatically grow as they became elderly. Thus, on average, what we will observe when studying average people is that on average the brain will worsen. This can become confusing for even very intelligent experts in the field. They make a logical fallacy that average outcomes represent what MUST happen.
I've encountered this with some clinicians and their beliefs around diabetes. Since the average person is a lost cause, they reason that diabetes is irreversible. This isn't based on any biological fact or reality. It isn't based on a set of biochemical laws or rules. It is purely based on the observation that basic average people have basic average outcomes. When we reference mechanism, namely cell turnover and receptor upregulation, the grey-scale that is insulin resistance/glucose intolerance-insulin sensitivity/glucose tolerance, what we find is that "diabetes" is an arbitrary point on an ever-shifting gradient. Not only is it entirely reversible, one can reverse the reversal. This is a function of lowest-common-denominator thinking and behavior. Over forty years ago, based on biology, we knew lifestyle was the cure for diabetes. Now, overwhelming case studies have proven it conclusively. Keep in mind that those irrefutable truths haven't changed the minds of average people thinking about average outcomes based on average effort.
Likewise in neuroscience, there is no biological reason why new brain cell growth would cease. Most researchers have confirmed this, although we do see that age-related afflictions will inhibit brain cell growth: https://www.nature.com/articles/s41591-019-0375-9. This is contended by no one really. Rates change. Different health issues affect it. Averages are bleak. None of that tells us what is possible or impossible. And it doesn't take a lot of effort to discover stories about elderly people who learn new languages, new skills, new athletic abilities, new careers, new adaptations to technology.
If, however, we insist on limiting beliefs, if we insist that average is what MUST be, then we have done nothing more than create self-fulfilling prophecies. We begin with the expectation that our brain must fall apart. Lo and behold, we stop challenging it to grow. We coach ourselves down to our starting expectation. That's at odds with numerous case studies. That's at odds with neuroscience. That's at odds with biology. The older brain can keep growing, as long as we seek to be more than basic and average.