I'm sure it is not worth much, but would love to know its approx. age.
"Stevens" shotguns were produced under various "Stevens" names as follows;
... Founded in 1864 at Chicopee Falls, MA as "J. Stevens & Co".
... In 1886 the name was changed to "J. Stevens Arms and Tool Co".
... In 1916, the plant became "New England Westinghouse", and tooled up for Mosin-Nagant Rifles.
... In 1920, the plant was sold to the Savage Arms Corp. and manufactured guns were marked "J. Stevens Arms Co." This designation was dropped around 1948, and only the name "Stevens" has been used up to 1990.
Since your 12 ga is stamped "J. Stevens Arms Co" it appears to be have been manufactured between 1920 & 1948. I couldn't pinpoint a definitive date of manufacture. I'm not sure I could even if I had the model number of the shotgun to go with the serial number as record keeping was not always accurate.
I have no idea what the value of this shotgun may be, perhaps other forum members may have an general idea.