How can the age of a tree be determined?
Depending on if you were there to plant this tree and watch it grow or other wise, the three common ways are to count manually, extract increment core, or calculate.
1) When you plant the tree, you can count its age every year.
2) Using a increment borer, you can figure out the closest approximation by counting the annual rings.
3) Calculating by growth factor.
According to Science ABC, "If you multiply the diameter of the tree by the average growth factor, you should get an approximate age, in years, of the tree. Due to the inherent variability of a tree’s growth, this can only be considered an approximation, and could be off by 3-5 years, at least."
The 'average growth factor' is found by researchers of the most common species, since different trees grow at different rates.