Your bones are continuously changing — new bone is made and old bone is broken down. When you're young, your body makes new bone faster than it breaks down old bone, and your bone mass increases. Most people reach their peak bone mass around age 30. After that, bone remodeling continues, but you lose slightly more bone mass than you gain.
How likely you are to develop osteoporosis — a condition that causes bones to become weak and brittle — depends on how much bone mass you attain by the time you reach age 30 and how rapidly you lose it after that. The higher your peak bone mass, the more bone you have "in the bank" and the less likely you are to develop osteoporosis as you age.