Age causes brain volume decline and intelligence loss, like distance causes tiredness in marathon runners. i.e. it can't cause anything, it's an abstract measurement - age doesn't cause rust, oxidation does. Distance doesn't cause tiredness, running does.

Increasing age and increasing decline can't mean that age causes decline, it shows one or more of: processes which cause decline keep happening, repair processes either don't happen or get worse, new generation either doesn't happen or gets worse.

(If worms were eating your brain, your brain volume would go down and your intelligence would go down as time passed and your age increased, it wouldn't prove that age caused your intelligence loss. Correlation isn't causation).