How strong is the evidence for Lifelong Learning?

The evidence linking continued mental engagement to dementia risk is unusually self-aware about its own biggest weakness -- reverse causation, where early undetected cognitive decline reduces a person's activity levels rather than inactivity causing decline -- and several recent studies were designed specifically to address it. That evidence is currently rated moderate (62/100), built on 5 published studies, including 1 meta-analysis -- a reasonable base, but not yet enough for full certainty.