Cognitive decline is not inevitable. 70% of cognitive ageing is modifiable — not genetic. Four mechanisms drive decline: amyloid accumulation (addressed by N3 sleep for glymphatic clearance), neuroinflammation (Mediterranean diet and DHA), vascular damage (aerobic exercise and blood pressure control), and synaptic reserve depletion (language learning, musical training, cognitive challenge). Prevention must begin in midlife — amyloid accumulation starts 15-20 years before symptoms. The evidence is specific enough to match interventions to mechanisms.