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.

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.