Decanting a young wineallows it to get oxygenated and breath, which will have the effect of releasing the aromas.
Decanting an old wineis a delicate operation which will have the benefit of being able to eliminate the deposit at the bottom of the bottle, but it can also damage the wine already weakened with age by providing too much oxygen.
(Keep the bottle upright 24 hours in advance, this allows the deposit to naturally fall down to the bottom of the bottle. If still needed choose a narrow carafe and only carafe the wine an hour before the meal)