Once a woman reaches the age of 30 she loses 1% of her bone mass every year thereafter (by the time a woman is 50 she will have lost 20%).
Bones are the framework for your body. Bone is living tissue that changes constantly, with bits of old bone being removed and replaced by new bone. You can think of bone as a bank account, where you make "deposits" and "withdrawals" of bone tissue.
During childhood and adolescence, much more bone is deposited than withdrawn, so the skeleton grows in both size and density. Up to 90 percent of peak bone mass is acquired by age 18 in girls and by age 20 in boys, which makes youth the best time to "invest" in one's bone health.
The amount of bone tissue in the skeleton, known as bone mass, can keep growing until the late 20s. At that point, bones have reached their maximum strength and density, known as peak bone mass. Women tend to experience a minimal change in total bone mass between age 30 and menopause. But in the first few years after menopause, most women go through rapid bone loss, a "withdrawal" from the bone bank account, which then slows but continues throughout the postmenopausal years. This loss of bone mass can lead to osteoporosis. Given the knowledge that high peak bone density reduces osteoporosis risk later in life, it makes sense to pay more attention to those factors that affect peak bone mass.