You might try calling Peavey, maybe they can tell you from the serial number.
Components inside the amp often have date codes on them. The potentiometers, transformers, and capacitors specifically. Here's one explanation I found with a quick Google of "EIA codes", http://users.eastlink.ca/~mitchlane/EIA_Codes.html
Obviously, the amp cannot be older than the components inside, assuming they haven't been replaced.