Updated: Jul 9, 2020
Bones grow best and strengthen well when you take enough calcium and Vitamin D3 as well as exercise with weight and have adequate body weight. Inadequate calcium and Vitamin D3 intake, lack of weight exercises, and being underweight may lead to thinning of the bones which in the early stages is called osteopenia and in later stages is called osteoporosis. When you are older and have osteoporosis your bones will break more easily with minimal trauma and sometimes they will break spontaneously.
One major source of calcium is dairy products such as milk and Yougurt where one serving (8 ounces) have 300 mg of calcium. You need at least 3 servings to get close to the minimal recommended amount of 1000 mg of calcium daily. Other foods rich in calcuim are green leafy vegetables, seafood and legumes. If you don't get enough calcium from food then you can take calcium pills. If you don't get enough Vitamin D3 then can take Vitamin D3 pills (about 5,000-10,000 units every week depending on your initial level of Vit D3). Vitamin D3 helps your body absorb calcium.
Weight exercises make the bones grow thicker and stronger. You don't have to use heavy weights, just what you can tolerate (but 3-4 times a week). Being underweight will make the bones grow thinner, so strive for your ideal body weight.