[trimble-alum]

It's possible to push oneself "harder" and regain function ("neuroplasticity"), as evidenced by stroke rehab.

This is why it's vital to do challenging mental exercises like crosswords puzzles, etc. ("use it or lose it.)

Generally though, the article makes sense because the common cell machinery of nerves (which don't divide as much as say intenstine or dermis cells) can be hampered by underlying issues affecting organelles within nerve cells, which then manifests as functional, quantitative deficit at the macro level.