At the risk of downvotes, I will state a whole bunch of [Citation Needed] BS, which is, nevertheless, empirically true & in Hardy's favor -

1. Cerebral atrophy affects left brain disproportionately more than the right 2. Math is predominantly a left-brain activity. 3. Ergo, short-term memory & dexterity with numbers generally degrades with age ( supposedly peaking at 25 & declining from then on).

Most math majors I've spoken to ( from prodigies to professors & people in between ) attest to the fact that they were much sharper when they were quite young. It isn't that they become dumber as they age, more like - they pick up a different kind of math to compensate for some of the other math that required them to exercise their symbolic manipulation skills at superior pace, which they have now lost.