Don't let @Intl Professor hear talk about old people like that.
We always think "old" is people 15 years older than we. He's old, for me. There are ways to cheat. After a decade or so viagra stopped working for me. But I just convinced by much younger wife to take Basis, by Elysium Health, an MIT spinoff by the head of their anti-aging research program. It is supposed to stimulate the metabolism at the cellular level. Not an advice, except to check out their site.
Basis by Elysium Health - Clinically-Proven NAD+ Supplement
I know, off topic, but perhaps useful to all of us who want, or think we need, even more brain power. (I think my writing here has improved, but hard to tell when you have such a smart audience to address.)