The year it was made is the second and third digit of the serial number. If it is 4 or 5 years old then you could have a failed dip tube, but usually there are other signs of dip tube failure besides lack of cold water. You should have tested the heater components before replacing them to determine which, if any, were defective. Until both thermostats and both elements are tested and the wiring checked to be sure all connections are made to the proper terminals, I would not advise replacing the water heater.