It all depends on the beer, and what you mean by "keep". It's not going to spoil or anything, so long as it's not infected, but it'll change over time.
Higher gravity beers tend to age better than lower gravity beers. Hop character tends to fall off over time, especially aroma. So something like an APA won't age as well as a stout might. Some beers, like a hefeweizen are much better drank young and don't age well at all.