If you have osteopenia, you have lower bone density than normal. Your bone density peaks when you’re about 35 years old. Bone mineral density (BMD) is the measurement of how much bone mineral is in your bones. Your BMD estimates that chances of breaking a bone from a normal activity. People who have osteopenia have a lower BMD than normal.
About half of Americans older than age 50 get osteopenia.
Sandy McDowell, What is Osteopenia?, Health Line (updated July 6, 2017), https://www.healthline.com/health/osteopenia