I went back and dug to find the exact source I was thinking of (I heard it in a freakonomics podcast long ago, took awhile to find).
https://freakonomics.com/podcast/reasons-to-be-cheerful/
https://www.inquirer.com/philly/health/Nature_vs_Nurture__Do...
The big thing is this is purely about IQ.
Think of it like folks being born at a certain IQ, and you can really only maintain it (exercise, love, healthy eating, sleep, etc.) You can't bolster it beyond what is innate. However, you (and your environment, typically our parents) can definitely cause decreases in your IQ (poor sleep, anxiety, poor nutrition, toxins like lead, etc.)
I found it pretty interesting, but also kind of obvious in hindsight. I would love to be able to increase my IQ, as would many others, but we all gotta just work the best to maximize what we got. Maybe tech / gene manipulation will make it possible in the future though.