I'm reading this in the same place you linked. It's a meta study of experimental studies, where experimenters ask otherwise sedentary people to be subject to "e

I'm reading this in the same place you linked. It's a meta study of experimental studies, where experimenters ask otherwise sedentary people to be subject to "exercise interventions" (it's in the title...). Observations weren't for years AFAIK, studies over 4 weeks were eligible and an effect was observed. Edit: as for your confounding effect, it doesn't strike at the heart of the study. The study finds that actually doing exercise improves your cognitive function. Not that there are "sharp" people and "active" people and these tend to go together. Whether or not you're sharp or healthy to begin with, doing "exercise intervention" will improve your cognition.