What is the age and value of a Stevens Arms and Tool Co shotgun?

If it is marked "J. Stevens Arms & Tool Co" it was made between 1886 and 1916 when "Arms & Tool" were part of the company name. Value will depend on the configuration and condition. Side hammers usually go as mantle decorations, especially if they have damascus barrels ($75 for single shots, $125-$150 for double barrels). Boxlocks (back hammer or hammerless) in good shape can bring $75 (single) to $250 (double) as shooters.