The entire title for this study is " Brain aging shows nonlinear transitions, suggesting a midlife 'critical window for metabolic intervention"
https://www.pnas.org/doi/10.1073/pnas.2416433122
I have not read the study yet, but one of the authors discusses it here:
Basically, they found that ketones (either exogenous or via diet) slowed or reversed aging. One reason is because of “neuronal insulin resistance”, and the ketones helped with that. They did find the diet to be better, and they theorize because of effects other than ketones but also the length in time when ketones were available (not that long for exogenous ketones).
She (the lead researcher) recommended lowering the glycemic load of your diet, starting in about your 40s.