Is there someplace on the internet you can look to determine the age and value of an American Gun Company side-by-side double-barrel shotgun that was your grandfathers?

No. These guns were made by Crescent Firearms and various Belgian manufacturers c. 1890-1930 and distributed by H&D Folsom of New York and also sold through the Sears catalog and retail stores. If you remove the barrels and look on the bottom of the chambers, you may find an oval around the letters ELG and a star. If there is a crown over this mark, it was made in Belgium from 1893 to 1914. If the mark is there but without the crown, it is older. If there is no ELG* proofmark it was made by Crescent. A hammerless Crescent SxS was made after 1903 and a side hammer gun was PROBABLY made prior to 1914. Its value as a mantle decoration is higher than its value as a firearm, usually between $150 and $250. If it is a total basket case, even less, and if absolutely like new a little more.