Babies are born without knee caps!
No not really, but they don't start out as the knee caps we know. Made of cartilidge that develops into bone through ossification, baby knee caps don't show up on x-rays. Baby knee caps are very soft, flexible and actually pretty darn durable. With all of the crawling and the toddling that includes endlessly bumping them, baby's knees really are like little heavy duty shock absorbers. Ossification (cartilidge hardening to bone) starts at about 3 years of age and continues until about age 6. What devine timing for this process of ossification to start after the rugid, rough and tumble months of learning to crawl, stand and walk.