Kids don't have kneecaps?

[Mikey]

I heard that kids under about age 8 don't have kneecaps....
I heard that it was just a cartilage that calcifies (is that a word?) later in life...

if that's true, does that explain why kids can crawl on their hands and knees without having any pain

[biohazard]

Kids do have kneecaps already at birth. However, they aren't always visible on x-ray, because they haven't ossified (become bony) yet. So in a way you are correct, but the kneecaps are there. The ossification process starts somewhere between 3 and 5 years of age depending on gender and genetics.

What comes to kids going on all fours I have no idea. My guess is, though, that the biggest factor is their size: maybe babies are small and soft enough to crawl around pain-free :)