Beer can age very well, but it tends to work best for darker, stronger ales and Belgian stuff, and is much more effective when the beer is bottle conditioned (ie has yeast in to enable secondary fermentation).
Standard stuff probably won't improve, and quality will be likely to drop off after a few years. Unlikely to kill you, though!
I can say that Fuller's Vintage Ale reaches its peak at around 5 years (attended a tasting a few years ago, very drunk afterwards!)
HTH,
Alex.