After the age of about 30, bone thinning is a natural process and cannot be stopped completely. Whether you develop osteoporosis depends not only on the thickness of your bones early in life but also on your health, diet, and physical activity later in life.

The thicker your bones, the less likely the bones are to become thin enough to break. Young women in particular need to be aware of their risk for developing osteoporosis and take steps early to slow its progress and prevent complications.
Plentiful physical activity during the preteen and teen years increases bone mass and greatly reduces the risk of osteoporosis in adulthood. If you eat a diet adequate in calcium and vitamin D and exercise regularly early in life and then continue with these healthy habits, you may be able to delay or avoid osteoporosis.
Eat a nutritious diet that includes adequate amounts of calcium and vitamin D. Both are necessary for building healthy, strong bones. The recommended daily calcium intake for adults up to age 50 is 1,000 mg a day. Men and women age 50 and older need 1,200 mg of calcium each day. The recommended daily intake for vitamin D is 400 to 800 IU a day for adults up to age 50. If you are age 50 or older, the recommended amount is 800 to 1,000 IU of vitamin D a day. The best source of vitamin D is exposure to sunlight. Vitamin D is vital for calcium absorption in bones and to improve muscle strength. One study showed that vitamin D may reduce an older person's risk of falling by 22%.
Take supplements if you are not getting enough calcium and vitamin D in your diet. Most doctors suggest daily vitamin D supplements for children and teens, starting by age 2 months. Talk with your doctor about how much and what sources of vitamin D are right for you and your child.
Get regular exercise. Weight-bearing exercises, such as walking, jogging, stair climbing, dancing, or weight lifting, keep bones healthy by working the muscles and bones against gravity.
Don't smoke. Smoking puts you at a higher risk for developing osteoporosis and increases the rate of bone thinning after it starts.
After osteoporosis develops, getting enough calcium and vitamin D, along with other healthy habits, can slow the process and reduce the chances of bones breaking. It's common for a person's diet to supply only half the calcium the bones need, so you probably need to take supplements. Your bones need vitamin D to absorb calcium. One study showed that vitamin D may reduce an older person's risk of falling by 22%.
Research studies do not agree about whether calcium plus vitamin D supplements can prevent fractures. Some studies show that calcium and vitamin D supplements reduce the risk of fracture. But other studies show little effect of supplements on fracture risk. The greatest benefit of supplements appears to be for people who have osteoporosis. Calcium and vitamin D supplements are recommended if you have been diagnosed with osteoporosis.
(Source: Webmd.com)