1st the people on HN are not a random sample of the population. For those in computer science, programming, and science, learning is THE job in a sense...and there is definitely evidence for use-it-or-lose it and the brain. So all the personal stories here won't tell us much about what happens to the average human as they age...and even then it will happen to us at some point.

Also, pontificating on how stressful it is for older people to be wrong without empirical evidence is rich.

Overall, the quality of HN posts about non computer/ML topics is quite poor.