This is sometimes called the fracture threshold. The more bone you have at peak bone mass, the more you can afford to lose before you are in danger of having a fracture. Like muscle, for example, putting bones under stress, or 'making them work', stimulates their development. And the simplest way to make them work is weight-bearing physical exercise. The longer you live the more likely you are to develop osteoporosis.
Osteoporosis affects both men and women, however it is more common in women.
Have you had an early menopause, ie. Women's Health Interactive in the United States has a comprehensive web site, including a form to help assess your risk of osteoporosis. There are things you can do now to maintain or increase strength of bones and help protect yourself against osteoporosis. These habits should start in childhood and continue throughout life. Keep up regular exercise - start good exercise habits in childhood, especially weight bearing, eg. walking, dancing Avoid high intake of alcohol, salt or caffeine, eg. coffee, tea and cola drinks.