If it happens at all, you usually get osteopenia after age 50. It depends on how strong your bones are when you're young.
Some people are genetically prone to it, with a family history of the condition.
You're also more likely to get it if you're a woman.
Calcium is the key to keeping bones healthy. Hormone changes that happen at menopause increase the chance for osteopenia for women. Men with lower testosterone levels have higher odds of getting it.