Bone isn't finished tissue you carry around — it's rebuilt constantly, and after about 30 the rebuilding slowly loses ground. For women, that slide accelerates around perimenopause and after. Calcium, vitamin D, protein, and load-bearing exercise are the four things that slow it down.
Most adults need somewhere around 1,000–1,200 mg of calcium a day. A cup of milk or fortified soy milk is roughly 300 mg, a cup of yogurt 250–300, an ounce of hard cheese around 200, and canned salmon or sardines with bones, tofu set with calcium, and cooked greens all contribute.
Calcium doesn't work without vitamin D, and most people in northern climates are low for half the year. That's one of the few supplements genuinely worth taking for a lot of adults — ideally after a test.
Weight-bearing and resistance training tell your body the bone is worth keeping. No amount of calcium replaces the signal that lifting sends.