From Anatomy and Physiology for Speech, Language, and Hearing:
pg.127
During early development, the lungs completely fill the thorax, so that they are not stretched to fit the relaxed rib cage. As the child develops, the rib cage grows faster than the lungs, and the pleural linings and increased negative intrapleural pressure provide a means for the lungs to be stretched out to fill that space.
The result of this stretching is greatly increased capacity and reserve in adults, but not in infants. Because the thorax and lungs are the same size, infants must breathe two to three times as often as an adult for adequate respiration. The adult's lungs are stretched out and never are completely compressed, so there is always a reserve of air within them that is not at the moment undergoing gas exchange.
pg. 132
As we mentioned earlier, adults breathe between 12 and 18 times per minute while at rest, but the newborn will breathe an average of 40 to 70 cycles per minute. By 5 years, the child is down to about 25 breaths per minute (bpm), and that number drops to about 20 bpm at 15 years of age. The adult has a considerable volume of air that is never expelled, but the infant does not have this reserve. In essence, the thorax expands during growth and development and stretches the lungs beyond their natural volume.
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