Mention bone density and most people envision an old woman, stooped and fragile—but a more accurate image might be your seemingly invincible teenager. Up to 90 percent of peak bone mass forms in girls by the time they're 18 and in boys by 20, marking the teen years as the most critical time to build up a lifetime "bone bank" (Archives of Pediatric and Adolescent Medicine, 2006, vol. 160, no. 10). Starting in middle age, bone density declines gradually—so the more your teen pays attention to her skeleton now, the less likely she'll be to develop osteoporosis, a painful and debilitating late-life disease characterized by weak and brittle bones.

Building blocks

This you already know: One of the best weapons against osteoporosis is calcium, the primary building block for bones. "Beginning at age 9 and throughout the teenage years, experts recommend 1,300 mg of calcium daily," says Karen Collins, RD, nutrition advisor to the American Institute for Cancer Research. She suggests three to four daily servings of "excellent sources of calcium," with milk products being the best unfortified sources. A cup of cow's or goat's milk, a cup of yogurt, or around 1.5 ounces of cheese all count as stellar calcium servings.

What if your teen is lactose intolerant, vegan, or just not into dairy? Don't sweat it: Plenty of calcium-rich, nondairy options also provide this much-needed mineral. A cup of fortified orange juice or milk substitute (soy, almond, or rice, for example) will supply your teen with more calcium than an equal amount of dairy milk. So will a serving (around 1 cup) of breakfast cereal fortified with at least 30 percent of the daily value for calcium. Tofu can also be a good calcium source, but the numbers depend on the way it's processed; look for tofu made with calcium sulfate. Also, help your teen load up on calcium-packed dark green leafy vegetables, beans, broccoli, and almonds.

Beyond calcium

But calcium isn't the only nutrient your teen's bones need. Vitamin D—often added to milk—helps bodies fully absorb and utilize calcium. If your teen gets sufficient sun exposure (at least an hour a day), she's probably getting enough; if not, experts recommend 800 to 1,000 IU of vitamin D daily. Copper, manganese, iron, zinc, and vitamins C and K assist in bone structure. Magnesium supplements taken during adolescence may also improve bone mineral density (Journal of Clinical Endocrinology and Metabolism, 2006, vol. 91, no. 12).

If your teen's diet includes a wide variety of healthy foods, he or she should have no problem getting all of these crucial vitamins and minerals. Foods are preferable to supplements because absorption is more dependable; but if your teen is a finicky eater or tends to skip meals, supplements are the way to go. Talk to your teen's doctor about a multivitamin to cover the basics, plus a calcium and vitamin D supplement to reach the recommended 1,300 mg. Spread out supplement intake during the day to improve absorption.

H.K. Jones is a registered dietitian and freelance writer based in Washington, D.C.