The scaling laws probably have something to say as well: twice the size means 8 times the mass, so adults are, on average, more delicate than children. There might be a similar undiscovered principle for cognitive function, but that's speculation on my part.
On the whole, though, I completely agree: there's a lot of ageism and resignation among adults about what is and is not within the abilities of the determined adult. Everything include language acquisition, learning to play musical instruments, mathematical creativity, and making scientific discoveries.
As a person transitioning into "unambiguous adulthood", but with plenty of youth left (I am only 30, for reference, and as creative as I ever was), this is super frustrating.