The aging of different organisms is mainly calculated by the special technique called the Radiocarbon dating.
In this technique the age of any specimen is calculated on the basis of the present amount of an isotope of a particular element.
Isotopes are the atoms of an element that have same atomic number but different atomic mass. For example in nature Carbon have three isotopes C¹² C¹³ and C¹⁴ . Among these C¹² is most stable and C¹³ and C¹⁴ are not stable. They slowly get converted into C¹² with time.
The relative comparison of fresh and old samples can show the difference on the basis of the concentration of C¹³ and C¹⁴.
If the concentration of C¹³ and C¹⁴ is low it point towards the aging of the sample.
Another method which simply solves the mystery is the principle of superposition.
It works in case of sedimentary rocks which are formed by deposition of various layers.
According to this principle the deeper is a layer more older it is.
The older fossils are the ones that are buried in deeper layer of the rock. The fresh dead bodies are present in the upper layers.