No, neuroscience agrees that there is a slowdown, as well as specific points at which different parts of the brain stop improving at the rate it used to. 35+ years old is widely agreed upon to be a point at which it is extremely difficult to form new neural connections at the rate it used to when you were 16.

No, you are not on track to become a vegetable the day you turn 25. Yes, it requires more and more investment, and literally noone in neuroscience disagrees with that.