Foresters determine tree ages by counting the growth rings of a severed tree stump or by taking a core sample using an increment borer. Still, it is not always appropriate to use these invasive methods to age a tree. There are noninvasive ways to estimate tree age in common trees where they are grown in a forest environment. There is also a way to roughly age a tree by using a "rule of thumb" for street and landscape trees.
A Tree's Growth Rings - Photo by Steve Nix