During the resting heart, that is, the number of times the heart beats per minute while the body is relaxed and at rest does not change significantly with age; But with age, there are changes in the ability of the heart to increase its rate during exercise and other moments of stress. Younger hearts usually usually speed up more faster than older ones when challenged. The maximum heart rate, the highest heart rate one can achieve as measured by a cardiac stress test declines gradually in adults as they age. The cause of this is slowing down of the heart's pacemaker cells.