A remarkable 78-year follow-up of 5,046 British participants revealed that childhood factors significantly predict dementia risk decades later. Low childhood cognition doubled dementia risk (HR 2.06), while pollution exposure, left-handedness, sleep disturbances, bedwetting, and emotional problems each increased risk by 38-106%. Among 121 dementia cases by age 78, the APOE ε4 gene and higher BMI showed female-specific associations with dementia. This unprecedented longitudinal study suggests dementia's roots may extend far deeper into early life than previously recognized. The findings challenge traditional views that dementia primarily stems from midlife factors, instead pointing to critical developmental windows. Sleep disturbances and emotional problems in childhood also predicted higher mortality rates across the lifespan. The research implies that early interventions targeting childhood cognitive development, sleep hygiene, and emotional wellbeing might offer novel dementia prevention strategies. However, as a preprint awaiting peer review, these provocative findings require validation. The observational design cannot establish causation, and some associations may reflect shared genetic or environmental factors rather than direct childhood influences on late-life neurodegeneration.