SUMMARYAge impacts the phenotype of all multicellular animals, but lifetime changes in physiological traits are poorly understood for all but a few species. Here, we describe a cross-sectional study of age effects on body composition, aerobic performance and ventilation in deer mice Peromyscus maniculatus. This species lives considerably longer in captivity (in excess of 5 years) than most laboratory rodents, and the adaptational biology of its aerobic physiology is well studied. Our deer mice grew throughout life,and, as is typical for mammals, their basal metabolic rate (BMR) and maximal oxygen consumption in exercise(V̇O2max) and thermogenesis(V̇O2sum)increased as power functions of mass. Age did not affect BMR, but we found abrupt decreases in growth rate, V̇O2max and V̇O2sum at approximately 485 days of age, and the mass-adjusted maximal aerobic performance of old mice (5 years of age) was 20%(V̇O2max) to 35%(V̇O2sum) less than that of young animals. Breathing frequency (f) and oxygen extraction (EO2) also declined with age but did not change abruptly. However, there were no consistent age-related changes in tidal volume (VT) or minute volume(V̇min) after accounting for the effects of mass and V̇O2sum. Age influenced several aspects of body composition (lean and fat mass). However,these changes were insufficient to explain the age-related declines in aerobic performance, suggesting that mass-specific oxidative capacity of lean tissue decreased with age. The performance changes we found could engender substantial reductions in the mobility and thermal tolerances of old deer mice. However, very few wild mice are likely to survive to ages where substantial performance decreases occur, so these declines are probably not subjected to strong selection in natural populations.