Long bone growth is very interesting because during long bone growth newly produced cartilage cells die and are replaced by bone cells. It is this production and replaced that constitutes bone growth.
At the ends of long bones are areas called "growth plates." Each long bone has at least two growth plates, one at each end. Growth plates are growing (new cell producing) tissue. During growth, growth plates produce new cartilage cells that divide and push older cartilage cells to the center of the bone. There, the older cartilage cells are eventually replaced by bone cells.
When the genetically governed growth in length and shape of a long bone is completed, then the growth plate tissues themselves are converted to bone. Thus, it is the very active and very critical growth plates that fuse at the end of the growth of a long bone.
When a bone has reached its full size, its growth plates are converted into bone. (BBC Science: Human Body & Mind)