According to WebMD, cardiac output is greatest during intense exercise and when a person is approximately in his 20s. The more intense the exercise, the more blood the body's muscle demand from the heart and, therefore, the cardiac output is normally greater than at rest.
Cardiac output is thought of by doctors as a combination of heart rate and stroke volume. Stroke volume is increased when the heart muscles strongly contract. Both heart rate and stroke volume are increased during exercise. The heart muscles also tend to contract more strongly when a person is young. Therefore, the amount of blood that flows out of the heart in one minute, cardiac output, is great in both cases.