The purpose of this study was to compare exercise measurements in yearling, two-year-old and adult Thoroughbreds using a standardised treadmill incremental exercise test. Peak oxygen consumption (V̇O2peak: 128.0 ± 2.1, 140.0 ± 2.1, 163.7 ± 3.4; ml/kg/min ± se, P<0.05), peak packed cell volume (PCVpeak: 0.50 ± 0.01, 0.58 ± 0.01, 0.64 ± 0.01litres/litre ± se, P<0.05) and the maximum number of steps completed in the exercise test (STEPmax: 7.7 ± 0.1, 8.1 ± 0.1, 8.6 ± 0.1; steps ± se, P<0.05) increased with age and degree of physical activity. Peak venous lactate concentration (LACpeak: 21.3 ± 1.5, 19.5 ± 1.7, 14.4 ± 1.7; mmol/litre ± se, P<0.05) and peak respiratory exchange ratio (Rpeak) were significantly higher in both groups of younger horses compared to the adult racehorses. Peak heart rate (HRpeak: 230 ± 2, 231 ± 3, 229 ± 3; beats/min ± se) did not change with age or training. The rate of change of V̇O2 between steps in the exercise test (V̇O2trans) was significantly lower in the adult racehorses at the highest exercise intensities. The slopes of the linear approximation between R (LinR bx), the natural log transformation of venous lactate concentration (LogLAC bx), and heart rate (HR bx) with velocity were significantly lower in the trained adult racehorses. The slope of venous lactate concentration normalised to per cent V̇O2peak (LogLAC per cent bx) was significantly lower and R breakpoint (R brkpt) normalised to per cent V̇O2peak was significantly higher in the trained adult racehorses. There was a more rapid decrease in venous lactate and a more rapid return to initial R values in the adult horses relative to the younger, untrained horses. No significant age or training effects were found in the remainder of the post exercise measurements. These results indicate that aerobic power and exercise capacity increased with age and training. Anaerobic power was already well developed even at a young age.