What is the effect of training the respiring (breathing) muscles?

During exercise there is an increase in physical activity and muscle cells respire more than they do when the body is at rest. The heart rate increases during exercise. The rate and depth of breathing increases - this makes sure that more oxygen is absorbed into the blood, and more carbon dioxide is removed from it.

The more you move your body the faster your heart pumps blood, and oxygen comes Into your lungs faster, doing this more and more will make it easier for you to breath and your lungs will be more open for air

The answer would be age 18.