The lungs lose some of their elasticity with age, and the thoracic cage undergoes some structural changes. This means that the respiratory muscles must generate more pressure in order to produce a given change in lung volume (this is called reduced compliance).
Lung compliance in a 20 year-old compared with a 70 year-old (source).
Calcification of the cartilage, osteoporosis, and weakening of the respiratory muscles reduce the ribcage’s ability to expand and contract.
The presence of antioxidants within alveoli (where gases are exchanged with the blood) decreases, while persistent inflammation within the lung increases. This can injure the alveoli, causing gas exchange to become less efficient. The result of these changes is a reduction in overall lung function and exercise capacity. Older lungs have a lower maximum rate of oxygen exchange and cannot expel air as quickly.
Age related change in forced expiratory volume in 1 second (the maximum volume of air that can be expelled from the lungs in one second) (source)