Genetically determined higher birthweight within normal ranges creates a measurable advantage for healthy aging that persists across the entire lifespan. Each one standard deviation increase in birthweight corresponded to 12-16% higher odds of extended healthspan, improved resilience scores, and 1.22-1.74 additional years of parental lifespan. The analysis of nearly 2 million European participants identified 26 specific mediating factors that channel birthweight's benefits into later-life health outcomes. These mediators span socioeconomic indicators like education and income (contributing 12-28% of the effect), behavioral factors including dietary patterns and sexual debut timing (10-30%), physical functions such as grip strength and blood pressure (8-43%), and cardiometabolic disease prevention (25-70%). This represents compelling evidence that early developmental advantages compound throughout life via modifiable pathways. The findings challenge the fatalistic view that genetic lottery determines aging outcomes, instead revealing that interventions targeting education, lifestyle behaviors, and disease prevention can amplify or potentially compensate for birthweight disadvantages. The research provides a roadmap for life-course interventions, though the European-ancestry limitation and observational design warrant broader validation studies.