There is only one way to tell how old a magnolia tree is, and that is by counting the tree's rings. Each year the tree puts on a new layer of growth, creating a thin dark ring between the new growth and last year's growth. By counting these rings you can determine how many growing seasons the tree has seen--in other words, how old the tree is. The trick is to find a way to count the rings without cutting the tree down to reveal them. Fortunately there is a way to accomplish this. It is a device called an increment borer.
Obtain an increment borer. This is a device for extracting a core from a tree. Check online or contact your local forestry department to locate this device.
Separate the three parts of the borer--the handle, the borer bit (which looks a little like a large screw) and the extractor.
Assemble the increment borer. This involves pushing the locking latch in the handle aside with your thumb and then inserting the square end of the borer bit into the hole in the center of the handle. Release the locking latch so it locks the two pieces together in a "T" shape.
Rub bee's wax onto the the threads as well as the shank of the borer bit so that it will penetrate the tree more easily and can be more easily removed.
Start on an uphill side of the tree, if there is one, and align the bit so that it will drill straight into the tree, at a height of 4 1/2 feet, with the bit horizontal with the ground. The goal is to drill into the very center of the tree.
Apply slight pressure and turn the handle of the bit clockwise until the bit begins to penetrate the tree. Once the bit penetrates the bark and enters the tree, open your hands and use your open palms to continue turning the handle of the increment borer clockwise until the bit has reached the approximate center of the tree.
Insert the extractor all the way through the hole in the center of the handle. The extractor will be in the inside of the bit. Insert the extractor with the concave side down (the open end facing the ground). Once it is inserted, turn the handle of the borer one half turn counter clockwise, until the concave side of the extractor is facing up.
Pull the extractor out of the borer bit. A core of the tree should pull out with it. Set it aside and remove the tree borer from the tree by turning the handle counter clockwise.
Count the dark rings on the core you have extracted from your magnolia tree. Count the number of rings from the core of the tree to the outer edge. The number of rings corresponds to the age of the tree.