We have a chain liquor store that sells single bottles and cans, which seems like a good way to explore styles by tasting a variety of commercial examples. However, the beer is not properly refrigerated during shipping and sits on open shelves at room temperature in the store. That means you might get a very stale sample that doesn't properly reflect the style.
Actually as I understand, hops make a pretty good preservative. So, I would agree that age may play a roll in the longevity of freshness, so too would the IBU characteristic of the beer. An IPA, for example, would have a longer shelf life than a porter. But unless that beer on the shelf is many years old, it should still be a good representative of the style — IMHO.
As mentioned, IPAs have some of the shortest shelf life due to the volatility of hop aroma ad it's susceptibility to oxygen deterioration. Porters actually have a fairly long shelf life. Dark malts have been known to show anti-oxidation properties.
i guess one benefit of the ontario beer monopoly is that the beer is almost always stored pretty well. the beer store (only sells beer and cider) keeps the entire store chilled to something like 60F, so you always feel a chill when you walk in, and so everything on the shelf is chilled enough to drink immediately. the newer/better LCBO stores have a section of the store with an automatic door that is entirely chilled like this for beer, white wine cider etc.
so the euro beers i get to taste, as ive said many times are the only thing worth buying. i know ive been spoilt by a wide variety of german lagers/pilsners always in good shape for an okay price.