Cognitive ability or performance? I can imagine that being in your 50s you would hit a sweet spot between declining ability to learn new things and a vast body of knowledge and skills already acquired.

Wow, I hope it isn't 50!

I think the 50's, 60's and 70's (and beyond) are highly correlated with the challenges we take on.

Children thrive on regular, daily, habitual challenges, mental, social and physical. But so do people in their 90's.

The amount of regression or hard limits on progression associated with aging, that are actually inherent to aging, are surprising small.

Although, reversing long term regressions gets more difficult. And we can run into impediments and unfortunate events beyond of our control at any age, but age makes us more vulnerable.