I thought it was well understood that your mental/physical potential peak when you are young and degrade thereafter? E.G. comparing 26 year old self to 86 year old self.

But a saving grace is the degradation can be combated with exercise both mental/physical.

I am not sure how much of aging is understood, so I hesitate to mention stuff like DNA damage.

I think that's a total misconception. I think the myth that "the brain stops developing at 25" was an apocryphal conclusion from a study where they simply stopped measuring past 25 and that either the same study or other studies found many brain faculties don't plateau, ever, even in advanced age. I also heard that it's highly variable from person to person how the brain develops, like some 8-year-olds had faster development in some regions than some 30-year-olds.

One thing's for sure though: we're still in the stone age of neurology.

Years ago we were taught that the brain could not grow. You had what you were born with and after your twenties it degraded.

Now of course we are aware that the brain has plasticity. It can rewire itself, grow, learn.

so primitive. but we're finding things out, from ultrasound surgery to cut out the addiction center of the brain, to influencing motivation/treating depression with magnetic/electrical fields with TMS and tdcs, we're ever so slightly making sharpened stones and crude axes of the stone age we're in

There's a difference between slowing down and losing cognitive abilities. It's one thing to not be able to solve very complex problems anymore and another to not remember how to use a toothbrush. I don't think anyone's really disputing that we're on average more capable in 20s-40s.