It has nothing to do with worn out muscles. These are young girls who don't know what they're talking about. I suggest that you talk to your doctor about it and get a bone density scan to find out if you have osteoporosis. Osteoporosis causes bone loss, and you can lose inches off of your height from it. You need to know if you have it so you can get treated. Don't stress out about it, though. Many, many women have it, I have it, and you can't die from it.
yes it is very possible because as you get older your muscles start to ware out and they shrink so that means that your body does also, but your bones stay the same size...crazy I know but that is what my doctor told my grandma(:
Yes this is possible. Your muscles ware out and therefore shrink. Your bones don't change though. Do not worry about it either though, everyone shrinks at one point. My doctor has told me this along with other people in my family. :D