The popular belief and “fact” that does the rounds is that babies are born without kneecaps! In fact, babies are born with kneecaps, they just haven’t gone through the ossification process – a process that turns cartilage into bone (check out Fun Fact Friday – Week 7 for more info about ossification). The kneecap, or patella, doesn’t show up on x-rays of babies and has led to this myth. Being the largest sesamoid bone (a bone that exists in the middle of a tendon) in your body, it takes somewhat longer than other bones to become “proper” bone.

It’s just another clever design feature! Imagine a toddler with fully ossified kneecaps, learning to walk with all the falls, trips and spills. Then once they’ve mastered walking, all the impulsive bouncing, dancing and careening into walls, coffee tables and anything else that dares to get in its way. The hard kneecaps would take the flack and break under many of the more drastic incidents, causing floods of tears and undoubtedly trips to A&E.

Instead, they are soft, rubbery and bouncy, taking all that a toddler can throw at them. This enables them to fall over, knees taking the brunt of the impact and get back up with a giggle, a smile and carry on to repeat the process again and again. By the time they reach somewhere between 3 and 5 years of age, the kneecaps will have undergone the ossification process and turned into “big person” kneecaps! For every year after that the knee caps will loose their bounce, the dancing and careening will lessen, and they’ll start to grumble and creak into old age.

http://health.howstuffworks.com/human-body/systems/musculoskeletal/babies-kneecaps.htm