Of course nothing stays constant - even " identical " twins become less similar by middle age ! Many cells have to divide so we can grow new skin, blood, bone and other tissues when needed. Our unique genome may be affected by errors ( mutations ) during division, which can cause disease like cancer. Even the iris and retina are susceptible. Many, if not all, human diseases are caused by a combination of genetic predisposition and environmental factors.
Each of us is not only unique but we also change with time - our cells, bodies, and minds ! Already there are tests which claim to help predict how long we will live - if we really want them - but until we do know more about ageing the key word is moderation in all things !
http://learn.genetics.utah.edu/conte...its/telomeres/
http://filipinaroses.com/showthread....y-want-to-know