Since there are so many different kinds of beer, it isn't too much of a stretch that they have a wide range of conditioning needs. Some are very much ready to drink right away. Some are definitely best drunk fresh to preserve (mostly) hop attributes. Others really benefit from cellaring, lagering, or both. See for yourself: Brew a decent size batch and just drink a glass every week for a while. You'll see which characteristics improve, decline, or evolve. And it will vary for each beer style and even for each individual batch.
Generally speaking, the more roasty or high alcohol, the more aging benefits the beer. The more hop aroma, the more time hurts it.