From our Forestry Specialists:
Estimating the age of a tree based on diameter has a lot of potential for error. Trees grow at different rates depending upon climate, overall health, local soil conditions, light availability, competition, etc. Especially with larger trees, I don’t think you can reliability draw inferences about tree age from just diameter.
For an accurate measurement of age, the best way would be to have someone do an increment bore in the tree. This takes a core of tree, about the size of a drinking straw, and allows you to count the growth rings for a better estimate of age. How close the core comes to the center of the tree will depend upon how accurate the age measurement is. If the landowner is interesting in finding the age of a tree, many arborist have increment borers and could be hired to bore the tree. I am sending separately an additional reference that may be helpful to you.