There are two reasons for this. First, as people grow older they tend to be less physically active so the muscles of respiration become weaker. Second, the lungs become less distensible, less compliant or stiffer with age. This is due to an increase in the amount of fibrous lung tissue. Hence, a maximal ventilatory effort does not expand or compress the lungs to the same degree in an older person as in someone younger.
Vital capacity tends to increase with height.
The reason is anatomical. A taller person has a longer thoracic cavity and larger lungs. Body weight does not appear in the equations. In other words, vital capacity is independent of body weight. If two people have the same gender, age and height the dimensions of their thoracic cavities and lungs, on the average, do not differ.