Please watch this short video.
http://www.revolutiongolf.com/home/video/1209810120001
While it is part of a golf instructional series, this is a great explanation of human posture and why your body is feeling good, or not, as you age. You can ignore the golf part, but there is a neat little "test" at the end that is not golf related that I suggest you try (in the last minute of the video). It's only 9 minutes long.