To quote Michael Jackson: "If you see a beer, do it a favour, and
     drink it. Beer was not meant to age." Generally, that is true.
     However, some beers that are strong and/or highly hopped must age to
     reach their full flavor potential.

     How a beer is conditioned and handled has a great affect on its
     shelf-life. Beer conditioned in the bottle or cask still contains
     live, active yeast and should be drunk as soon as possible. Most
     larger scale, commercial beers have been filtered or pasteurized to
     remove/kill the yeast and stabilize the product for the longer
     storage times encountered in the retail world. In any case, stored
     beer should never be exposed to heat or strong light.