Oldest homebrew i've tried to date was 3 1/2 years old, and even though it was a good quality Belgian strong dark ale, it had certainly seen better days. Having said that, it took an entire year in the bottle to become 'good'.
As opposed to certain simple kit lagers i've brewed that were ok after a week, good after a month, better after 2 months, then went downhill.
It's been said in the posts above, but certain beers will age better than others.
Naturally, stronger beers will store better, and often develop with age.
Particularly hoppy beers will tend to mellow with age too.
As with most dark beers, seem to stand up quite well to extended periods of bottle conditioning.
Going by these things, one would have to say a beer of standard strength, considering low hops and pale in colour, is best drunk now, rather than left to condition.
Having said all of this though, it was helps to have a good place to condition beers anyway. Somewhere devoid of light, with a constant and low temperature and humidity. A 'cellar' of sorts really.
