In childhood and early adulthood, bones store calcium faster than they release it, which leads to growth. Around age 30, bones reach their peak mass, and the rate of calcium storage slows to the rate of calcium release. Peak bone mass is usually kept well into the 40s. Afterwards, as people lose reproductive hormones, bones release calcium faster than they store it. This weakens them, and increases the risk of fracture.
Some people lose calcium faster than others, and get osteoporosis.
Although many other factors may lead to weak bones, loss of hormones is the most important. After age 40, women lose hormones faster than men. On average, the loss of reproductive hormones happens much faster in women than in men. For this reason, women are more likely to get osteoporosis than men.