Although feeling younger has been found to be related with various health benefits, associations are complex, and most prior studies have used self-reported health indicators rather than objective, performance-based health measures and biomarkers. We investigated between-person and within-person associations of subjective age with pulmonary function (peak expiratory flow), a biomarker associated with mortality risk. Data from the German Aging Survey were used, with a sample of 9,782 middle-aged and older adults assessed up to four times over 9 years. Longitudinal multilevel regression models controlling for socio-demographic and health-related measures revealed predictive effects at the between-person level and at the within-person level, and in both directions. At the between-person level, study participants who felt overall younger showed better pulmonary functioning, and those with overall better pulmonary function felt younger. At the within-person level, on measurement occasions when participants felt younger, they exhibited better pulmonary function, and on measurement occasions when participants’ pulmonary function was better, they felt younger. Our findings thus suggest that there are meaningful between-person and within-person associations between pulmonary function and subjective age. Even subtle changes in biomarkers such as pulmonary function may affect how old people feel, which has consequences for their behaviors, health, and well-being.