Trees are sometimes aged by the stem diameter size, and whilst this measurement can give an estimated guess, it is not accurate. Weather conditions have an impact on trees and their growth rate every year. For an example a long warm summer with enough rainfall will be a good growing year for a tree, whereas a hot summer with periods of drought will stress and minimize the growth for that year.
The way to correctly age a tree is by cutting through the base of the stem and counting the number of growth rings across the stem cut. By doing this you can also determine the good and the poor years shown in the width between each ring.
This of course is not recommended because it will kill the tree. It should only be done if the tree has to be removed, falls over or dies.