The timing of when biological sex differences emerge in cardiovascular function has remained surprisingly unclear, despite decades of research showing men and women have distinct heart disease risks and treatment responses. New electrocardiogram analysis of nearly 62,000 children reveals these fundamental differences don't exist at birth but develop precisely during adolescence. Researchers applied an electrocardiographic sex index originally developed for adults to pediatric heart rhythm data spanning ages 0-18 years. This computational tool analyzes multiple ECG features simultaneously to create a continuous score reflecting biological sex characteristics in heart electrical activity. The findings show striking developmental patterns: young children of both sexes cluster around identical neutral scores near 0.5, indicating no meaningful cardiovascular sex differences in early life. However, beginning around age 10-12 and accelerating through teenage years, boys' scores increase while girls' scores decrease, creating widening separation that stabilizes in late adolescence. This divergence occurred consistently across racial groups, suggesting a universal biological mechanism rather than population-specific effects. The research provides the first quantitative timeline for when sex-based cardiovascular differences emerge, directly linking these changes to pubertal development rather than genetic programming present from birth. For longevity-focused adults, this work illuminates why cardiovascular risk assessment and treatment strategies increasingly emphasize sex-specific approaches. The findings suggest hormonal changes during puberty fundamentally rewire heart electrical systems in sex-specific ways that persist throughout adulthood. Understanding these developmental origins may inform more precise cardiovascular prevention strategies, particularly for adults approaching hormonal transitions like menopause where cardiovascular risk profiles shift dramatically.