For a young tannic wine (like a Cabernet Sauvignon or a Nebbiolo for example), decanting the wine infuses it with more oxygen than is possible when the wine is served directly from the bottle. Exposure to oxygen helps the wine soften and develop – and can turn up the volume on the aromas. In fact, many wine decanters are made in shapes that serve to get as much oxygen into the wine as possible.
Should old wines be decanted? Actually, many wines that are well-aged need to be drunk fairly quickly so that they are not exposed to too much oxygen. A long-aged wine has already had plenty of time to soften and develop, and sudden exposure to too much air will flatten and deaden it. Sediment can often form in older bottles of wine, but the best way to remove sediment is to pour the wine through a fine-meshed strainer rather than decant it.
So, at the end of the day, should you decant your wine? It mainly depends on the grape and the vintage. For wines that are big, tannic and perhaps a bit tight (e.g. highly concentrated without much fruit flavor or aromas) – go ahead and decant away! For anything else, it’s a more a matter of presentation than necessity.