An adult man's body ages and loses its youthful vigor. But all the individual atoms that make up the body remain in like-new condition. Atoms never wear out or degrade.
An individual atom in his body remains the same indefinitely, with the same number of protons in its nucleus. The electrons do not slow down or deteriorate. The fundamental properties of an atom: its atomic number and mass, do not change over time for a single atom.
Similarly, individual molecules of simple compounds like water (H2O) do not degrade or lose their quality over time at the atomic level. The chemical bonds between the hydrogen and oxygen atoms in a water molecule will remain the same long after they depart from the man's body.
A chemical reaction inside the body will not change any of the atoms within it. The atoms will remain the same before, during, and after any chemical reaction.
It is impossible to look at an individual atom and determine its age. There is also no way to see that one atom has more entropy than another. Atoms of the same element will look the same, regardless of
age.
You can look at two different men and estimate which one is older based on their appearance. But you cannot do this with the individual atoms in each man. At the atomic level, both men would appear to have the same age.
If we want to take this thought deeper, we could consider how the form of man changes (his body ages), but his spirit within is eternal (like the atoms that never fade away). God made both. And the name he gave to the first man was Adam, which sounds like "atom". He gave us a temporary shell (the physical body) for a trial life which gives us a short time in this world to make our choice of free will. God also made all the atoms, which never age or break down, but appear to last for ever. And he made our soul to be the same, which will continue to exist without an end. Let us choose wisely here, so we may have bliss where we go next!
Even after the form of your cover returns to dust, from which it also came, the individual atoms of your body will remain in the same state as they were before you were born. Each individual atom that was once part of your physical body will appear exactly the same as before you walked on Earth. Atoms are virtually indestructible, unless God revokes the laws of nature. You also, though your body may die, will never cease to exist.
And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell. Matthew 10:28 KJV
Radioactive Isotopes in Human Body
Stable elements, of which the human body is almost entirely made from, do not decay over time. While a body might be contaminated with radioactive isotopes, as from the Three-Mile Island nuclear disaster, these are not required for human health.