What year was maytag refrigerator model rtd1900 aal made?

That is BS that the serial number is needed to age an appliance. I don't know why that Appliance 411 site requires the serial number. If the serial number were needed to date a particular model, then these manufacturers would still be using the same model numbers over and over, which they don't. Just try plugging an old model number into a search engine, and all that will appear are sites for parts and blogs detailing problems. On the contrary, when you plug in a current model (within 2 years) you'll find the model still for sale.