As we age, our physical functioning deteriorates. This is most likely due to reduced efficiency in basic cellular processes. One cause of this is thought to be

Since this is not my area of expertise, I searched Google under the key words "cellular aging process" to get this:

http://www.google.com/search?client=safari&rls=en&q=cellular+aging+process&ie=UTF-8&oe=UTF-8

In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.