Unfortunately, there is only one way to determine the age of a tree... and that is to chop it down and count the rings.
However, there are other less invasive techniques. For example, if a branch is pruned, count the rings of that branch and notice the thickness of that branch. Now compare the thickness of the branch to the trunk, and estimate the age.
I have seen Junipers found in the harsh environment of the rocky mountains which are atleast 100 years old and yet just 10 inches tall, whereas the same species growing in a park maybe 30 years old and 8 feet tall. Environment plays a huge role in age.
In my opinion, age should not be a deciding factor of whether one bonsai is better than the other. In fact, the Japanese never emphasize on the age of a tree... they focus on design.
Check out the below link.
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