Can improve. Resistance exercise and weight bearing exercise stimulate bone growth and improve bone density. Non weight bearing exercise such as bicycling and swimming are great exercises but won't improve bone density like resistance and weight bearing exercise.
Incr bone formation. Evidence shows that exercise help build and maintain bone density at any age. Studies have seen bone density increase by doing regular resistance exercises such as lifting weights for 20mi, 2-3 times a week. This type of weight-\bearing exercise appears to stimulate bone formation via bone stimulation through muscle contraction. Space zero gravity is an example of how bone density can be diminished.