Background <p>Cognitive function is a fundamental capacity essential for maintaining independence and performing complex tasks in daily life. Cognitive abilities gradually decline with age, potentially leading to dementia. Evidence suggests that female reproductive factors may influence cognitive function in later life through various mechanisms. However, the relationship between age at first birth (AFB) and cognitive function requires further investigation.</p> Methods <p>This study utilizes data from the 2011–2014 National Health and Nutrition Examination Survey (NHANES), including 1,057 female participants. AFB data are obtained from reproductive health questionnaire. Cognitive function is assessed using the CERAD Word Learning Test, Animal Fluency Test, and Digit Symbol Substitution Test. Multiple linear regression, smoothed curve fitting, threshold analyses, and subgroup analyses are conducted to evaluate the association between AFB and cognitive function.</p> Results <p>AFB is significantly and positively associated with cognitive function after adjusting for covariates. Women with AFB ≥ 20&#xa0;years exhibit a 34% lower prevalence of cognitive impairment and significantly higher cognitive test scores compared to those with AFB &lt; 20&#xa0;years. A nonlinear relationship is observed, with the positive effect of increasing AFB on cognitive function being more pronounced before age 21. The association between AFB and cognitive function is stronger among individuals without a history of stroke and those with moderate to high levels of waist-to-height ratio (WHtR) and weight-adjusted waist index (WWI).</p> Conclusion <p>Later AFB is associated with a reduced prevalence of cognitive impairment and improved cognitive outcomes, particularly when the first birth occurs after age 21. Considering WHtR and WWI may further optimize the protective effects of AFB on cognitive health. These findings underscore the importance of reproductive timing for long-term cognitive health.</p>