A baby is born with zones of cartilage in its bones that allow growth of the bones. These areas, called growth plates , allow the bones to grow longer as the child grows. By the time the child reaches an age of about 18 to 25 years, all of the cartilage in the growth plate has been replaced by bone. This stops the bone from growing any longer. Even though bones stop growing in length in early adulthood, they can continue to increase in thickness throughout life. This thickening occurs in response to strain from increased muscle activity and from weight-lifting exercises.
Context:
John was a relatively thin and bone-y child when he was younger. He would often get bruises and was prone to breaking bones. As an adult however he does lots of weight lifting and hasn't broken a bone yet.
Q: Will lifting weights likely increase or decrease bone thickness?