As mentioned in a previous article giving a review of osteoporosis, its causes and prevention strategies, physical activity places exertional forces on bone, which promotes bone growth and strengthening. This seems to be particularly important in your early 20s; keeping active during this time may reduce the risk of osteoporosis later in life.
http://www.medicaldaily.com/news/20120214/9112/osteoporosis-exercise-physical-activity-bone-disease.htm