Wrist-worn accelerometer data from 8,661 adults tracked over nearly 7 years reveals striking age and sex differences in how physical activity protects against cardiovascular death. High-intensity exercise provided 53% mortality reduction in 45-year-olds but only 25% in 75-year-olds. Women gained greater protection from exercise intensity (55% risk reduction) while men benefited more from exercise volume, though with diminishing returns. Benefits weakened substantially in people with higher waist-to-height ratios above 0.6. These findings challenge the one-size-fits-all approach to exercise prescription that dominates current guidelines. The data suggests younger adults and women should prioritize vigorous activity like running or cycling, while older adults and men may achieve better cardiovascular outcomes through longer durations of moderate activity like brisk walking. However, this preprint awaits peer review, and the observational design cannot establish causation. The results represent an incremental but meaningful advance toward personalized exercise medicine, potentially informing future guidelines that account for individual characteristics rather than applying universal recommendations across diverse populations.