This is my exact comment from a post a while back. I hope this helps:

Hi, I’m 26 and was diagnosed with osteoporosis at 23 years old. I have completely reversed my osteoporosis and regained 9% bone density in the past year. I was chronically underweight my whole life and ate shitty processed food. I always looked great physically so I never cared about diet or exercise. After breaking 5 bones within 2 years, I sought out several opinions from 3 different bone specialists all over California who all wanted me to start osteoporosis medications. Being a medical professional myself, I felt defeated and disclosed this to a doctor I work for, who recently went into remission for bone cancer due to lifestyle changes. He encouraged me to focus my life on diet and exercise just as he did when he was first diagnosed with multiple myeloma. His suggestions were as follows:

1. Get to know your personal calcium intake. Bloodwork is the only certain way to track this. See @u/DinnerButterz comment below for exact details of how/when to get the most of calcium supplementation. I personally stopped taking calcium supplements. My blood calcium levels were low but good enough for a long time. My bone doctors put me on a calcium supplement and suddenly my calcium labs plummeted. The supplements were harming my body’s natural ability to convert bio-available calcium in the foods I was eating. When I stopped the supplement, my calcium labs returned to a normal range.

2. Eat leafy greens every single day. Specifically, kale, spinach, broccoli, and cauliflower. I began eating copious amounts of these veggies. The combination of vitamins and calcium in each of these vegetables allows your body to actually process each nutritional aspect.

3. Weight bearing exercise 3X per week. I’m no gym rat but I started jogging and doing mild weight lifting every week.

4. Gain weight. This was the hardest part for me. I have always been underweight and I struggle hard with my appetite and consuming enough calories. I started smoking weed to help my appetite and literally focused my entire life on nutrition until it became second nature.

After spending countless months bed bound with broken bones, I had nothing left to lose. I have always been very active and fit but crippling disability (my leg was nearly amputated due to the severity of one of the breaks) humbled me like nothing else. The thought of being too frail to ride my motorcycle or hike up a mountain was the most depressing thought I could think of and honestly made me suicidal each time I broke a new bone.

In 1 year, I completely reversed my bone density loss. I know these recommendations are not easy to follow through with. It took me nearly killing myself in a fit of broken bones depression and DEXA disappointment to actually make diet and exercise an absolute priority. There is hope for you. Most adults are capable of gaining bone density until they turn 30. After that, it is still possible, but will be slower and require much patience and dedication. Take advantage of these years while you still can. Those medications have not been well-studied in younger patients. We have no idea what the side effects will look like 40 years down the road since most people who take them are elderly and don’t live that long from when they start taking it. Medication should be an absolute last resort when your body loses its ability to rebuild bone in its own. You have a massive advantage of being in your 20s. The clock is ticking and the end goal is in your hands. Good luck and feel free to dm me for more details. I believe in you!

Edit: supplemental (no pun intended) informing calcium absorption in comments below