I think that as an adult, to maintain flexibility it takes more work to do it. The nature of our bodies as children with the muscles and the still-developing connective tissue helps kids be flexible. BUT as adults, as long as we focus on it, we can still stay flexible. I also believe that some of it has to do with how much more active kids are, and the more active you are, the warmer your muscles, and the more flexible you are. If you try to touch your toes when you've been sitting on the touch for a couple hours, it you will be less flexible than if you tried it after going for a long walk.