This is more or less right... more accurately, as one's heart rate increases the ratio of fat calories burned to carbohydrate calories burned decreases. However this is offset by the fact that the rate at which we burn calories increases at higher heart rates. Eg at 128 BPM the 25 year old would say burn 10 calories/min of which 6 may be from fat (60%). At 140 BPM he is likely to burn 12 calories 6 (50%) of which may be from fat.

This is fairly accurate in the early stages for inactive people taking up an exercise regime...