Age is a very important parameter for many regression equations of physiological indicators, and it is an absolutely scientifically proven way.

We don’t use only age when we calculate Stiffness index, we use AGE + many other measured parameters because it increases the accuracy.

For example you can check in PubMed – ALL Body composition equations (for Fat mass, FFM, Muscular mass, Total body water, ECW, ICW ..) have AGE as a parameter because it increases accuracy of the results.

Fat mass also is related to the age and it is absolutely right.