Many brewers will buy these bottles back from a retailer a full retail price, so there is no reason to keeping old beer on the shelf.
But, because there are no standards or requirements for indicating beer age, each brewery employs its own method for dating its beers.
The information on this website will, however, usually produce a reliable manufacturing date range for a majority of American utilitarian bottles manufactured from the early 1800s to the mid-20th century.
Using physical, manufacturing related diagnostic features, most utilitarian bottles can usually only be accurately placed within a date range of 10-15 years (i.e., 1870-1880 or 1885) . Like many industries making the leap from manual craftsman production to industrialization and automation, technological advances in bottle manufacturing were not immediately accepted by glass manufacturers or their workers.
(Regrettably, a large number of breweries don’t offer any date stamps whatsoever, leaving you in the dark about the freshness of their products.) One of the most difficult tasks in checking beer freshness is locating the date code. Breweries such as Sierra Nevada, Great Divide, and Odell clearly print the date code on a specific section of the label, which is often the same for all product lines.
If you can’t find the code on the label, check the bottle or can.