Probably the most important thing in aging beer is realizing what types of beer age well and what types don’t. First, let me say this: professionally produced beer won’t spoil. There may be a “best by” date, but there’s no harm in aging anything and everything. That being said, not all beer will age well.
All beer will change with time. Yeast and the other microorganisms in your beer are living creatures – as time goes by, they will produce different biproducts that will change the flavor of the beer, for better or for worse.
A good general rule is that high-alcohol beers age well. The high alcohol content means two things. First, the large amounts of alcohol will protect the beer, killing off many of the bad flavors associated with a bad beer. Second, the high alcohol is (usually) the result of a large amount of base grain (more base means more fermentable sugars which means more alcohol) and these malty flavors often age well (I say high alcohol “usually” results from more base grain because this is generally the case with **cough** real **cough** beer; the Budweisers of the world are known for adding in extra sugar to increase the alcohol content of the beer without adding any body to it, keeping it light and tasteless).
A good rule of thumb is that 10% ABV and higher will age well.