Background and Purpose: We evaluated the influence of age on the association between sex and the incidence of stroke or transient ischemic attack (TIA) using a population-based cohort from Ontario, Canada. Methods: We followed a cohort of adults (≥18 years) without prior stroke from January 1, 2003 (cohort start date) to March 31, 2018, to identify incident events. We calculated hazard ratios (HRs), in women compared to men, of incident stroke or TIA, adjusted for demographics and comorbidities, overall and stratified by stroke type. We calculated piecewise adjusted HRs for each decade of age to evaluate the effect of age on sex differences in stroke incidence. Results: We followed 9.2 million adults for a median of 15 years and observed 280,197 incident stroke or TIA events. Compared with men, women had an overall lower adjusted hazard of stroke or TIA (HR, 0.82 [95% CI, 0.82–0.83]), with similar findings across all stroke types except for subarachnoid hemorrhage (HR, 1.29 [95% CI, 1.24–1.33]). We found a U-shaped association between age and sex differences in the incidence of stroke or TIA: compared with men, the hazard of stroke was higher in women among those aged ≤30 years (HR, 1.26 [95% CI, 1.10–1.45]), lower among those between ages 40 and 80 years (eg, age 50–59, HR, 0.69 [95% CI, 0.68–0.70]), and similar among those aged ≥80 years (HR, 0.99 [95% CI, 0.98–1.01]). Conclusions: Overall, women have a lower hazard of stroke than men, but this association varies by age and across stroke types. Recognition of age-sex variations in stroke incidence can help guide prevention efforts to reduce stroke incidence in both men and women.