I need to know the age of my Maytag refrigerator. Model # MTB2156DE.

Model means little, serial number is what is needed.

Here is an appliance date finder: https://dl.dropboxusercontent.com/u/8010163/Appliance_Date_Finder.pdf
I hope it helps, but you will need the serial number. If you happen to be lucky and have a unit with the R-12 refrigerant you should clean it and keep it. It will last for 50 years or longer and work on much less energy. You need to wash out the coils under the unit at least once or twice a year and keep it clean inside the freezer coils, too.