Use the serial number of a John Deere tractor to determine the age of the machine, but you also need the model number to search on the company's website. Find this on the metal plate on the rear of the tractor's frame.
The John Deere website has a look up for those seeking out the age of a tractor. These are PDF files sorted by approximate age. The first column has the model numbers in order, and the third column has the serial numbers. By using the model number to find the serial number, you can find the year the company manufactured the particular tractor.