We normally think that we need to cut a tree down & count the rings to estimate its age, however…
The girth of a tree can be used to estimate its age. Very roughly a tree will increase its girth by an average 2.5cm in a year. Simply measure around the trunk of the tree (the girth) at about 1m from the ground. Then divide the girth by 2.5 to give a rough age in years. A tree with a 50cm girth will therefore be about 20 years old.
The rate of growth will, however, differ according to what type of tree it is. Some types of tree, such as oak and beech, will grow quite slowly, and so you should divide the girth measurement by 2. Pine trees however are very fast growing, so you should divide the girth measurement by 3.
Other factors will affect the growth too, such as how close the tree is to other trees. If it’s on its own (like most of ours in Headington Hill Park), it has the space to grow faster than trees in a wood. (An average woodland tree will only increase its girth by about 1.25cm per year!)