In any bone, there are two competing cycles: breakdown and regeneration. Specialized cells called osteoclasts absorb worn-out bone cells, removing them constantly from the bone. Simultaneously, other cells called osteoblasts are taking calcium from the blood and redistributing it upon the bone. This process occurs throughout your life.
When you're young, you're building bone faster than it can be replaced. Until about the age of 20, your bones are focused primarily on one thing: getting bigger. Even after you've reached your maximum height, your bones continue piling on the calcium in an effort to get thicker and denser.
When you get old, however, your osteoblasts (bone-builders) can't keep up with the osteoclasts (bone-removers). When an older adult suffers a bone fracture, the body directs more resources toward the break, but the bone itself is already involved in a losing cycle of bone removal and replacement, with more being removed than replaced. This is why it takes older people longer to heal.
And this brings us to children. In a way, children's bones are always behaving like they're healing -- you couldn't stop those bones from growing if you tried. When a child breaks a bone, the body diverts additional repair cells to the location of the injury. The bone, however, is already engaged in a supercharged rate of growth. It's like having five people working together to knit a small scarf and adding 10 more people to help out after a stitch gets missed.
Most of the time, broken bones are but minor setbacks for children, and there's barely enough time to inspect all the cool signatures collected on their casts before they're back to 100 percent.
See the next page for lots more information on bones and the skeletal system.