A simple method to give you a rough idea is measure from the ground up to the trunk at chest height, about a yard(1m) then measure around the circumfrance. Each inch (2.5cm) should represent one year of growth (100in=100 years etc.
Note! calculations will be more acurrate if the chosen tree has been growing in an open area rather than a dence woodland, as woodland trees tend to grow tall and slender as they compete for light. Have fun Tbird+
Thanks for that Thunderbird. My grandaughter was asking me how old the oak tree was in our garden and I had no idea of how to age a tree. We will have fun doing what you've suggested.