Previous studies of the effect of age on maximal aerobic capacity (maximal rate of oxygen consumption, VO2max) in horses have only grouped horses (young, middle-aged and old) for statistical analysis. Those experiments were not designed to determine a break point due to age. The purpose of this study was to utilize data collected over the last 15 years to test the nature of the association between age and VO2max, and secondarily, to determine whether there is a ‘breakpoint’ in the age-related decline in aerobic capacity. Data were acquired from 50 unfit Standardbred mares ranging in age from 4 to 29 years, and were used in a retrospective regression analysis in order to characterize the age-related decline in VO2max. All VO2max values were measured using the same incremental graded exercise test protocol. The data were analyzed using a curvilinear regression analysis predicting VO2max from age, and the break point was determined using univariate simple contrasts. There was a significant decline in VO2max with age (R2 = 0.554; P < 0.001). The data analysis demonstrated a notable downturn in cardiopulmonary function between the ages of 18 and 20 years (P < 0.001). Such results indicate similar responses to age between humans and horses. These results have implications for improving training methods in active older horses.