For a rough estimate, use the diameter of the tree's trunk in centimeters. The number is an approximation of the years the tree has been alive. For example, a eucalyptus with a diameter of 30 centimeters at breast height is approximately 30 years old.. Measure from the base of the eucalyptus trunk up to 54 inches. Mark the level on the tree with a piece of masking tape. This is considered breast height.