In the world of wonderful spirits and aged liquors, the older the drink the more mature and flavourful it is. The same rule does not apply to most beers. In fact, beers are probably the most fragile of beverages and require a fair bit of care in preservation because the consumer is perennially at the risk of drinking either a very flat-tasting beer or a skunky one.
Even a pasteurised and triple cold-filtered brew does not take too long to go sour if it has been carelessly kept at home. Beer sitting out at room temperature, even for a few months, may start to degrade and become stale. What’s worse is that you have no idea if the shop owner transported the same beer cold and later allowed it to get warm. Cold storage slows oxidation in beer. Even the tightest bottling system will allow minute amounts of air into the bottle, which, over the months, will start to destroy the beer and give it a horribly damp flavour that requires copious amounts of Listerine to get rid of.
While buying beer, ALWAYS look for a date that will indicate its freshness. Some bottles have a “packaged on” date while some others have a “best before” date. Avoid buying beer that is kept in direct light, which allows damaging UV rays in. Both excessive direct sunlight and heat can contribute to the stale flavour, which is a by-product of the delicate hop oils spoiling. There is a reason why most beer bottles come in coloured bottles: the tinted glass acts as shades to ward off powerful rays.
Also beware that as much as discounts on beer seem alluring, no store will randomly offer a discount unless a perishable product like beer is close to its “sell-by” date. That said there are few beers that challenge all of the above. Stronger ones like Belgian ales, stouts and Trappist ones last longer and in fact gain depth of character when kept for a few months. Highly hopped brews tend to benefit from a longer shelf life due to the preserving quality in the hop oils. But unless you’re having one of these, keep it in mind that the younger the beer, the fresher it is.