There has been lots of confusions and questions raised regarding whether we should decant a bottle of wine before serving, now this is a very broad question, experienced drinker would have different answers for different types of wine.

I found this piece from the official website of Chateau Margaux giving a clear view of this and would like to share with you, hope you will enjoy reading:

"The main aim of decanting red wines is to separate the wine from its sediment which may have formed in the bottle over time. This deposit is essentially composed of tannins made insoluble by the chemical reactions responsible for the ageing of the wine. The operation of decanting causes a certain oxygenation which is often beneficial to young and powerful wines, but which can be detrimental to very old wines. So how should we proceed ? It is unadvisable to set strict rules, even for a wine produced from one estate, since they do not take into account the incredible diversity of vintages. It is not always necessary to decant young wines, as generally there is not yet any deposit in the bottles. But letting them breathe a little can often help them to open out, in other words, to give off their aromas. You can choose either to decant them or to leave them a while in the glasses before drinking them, thus giving yourself time to admire their colour, to smell them a first time, then a second time, to listen, to talk, to anticipate the pleasure that awaits.

Once wines have left a sediment in the bottle, whatever their age, it is better to decant them.

What a pity it would be to serve a great wine that is cloudy. Even if its taste is not affected by the deposit, the pleasure of drinking it is not quite the same. You should simply pour the wine delicately into a decanter, or even into another bottle, over the flame of a candle. You can then see the deposit slide up the side of the bottle into the shoulder and then into the neck of the bottle. As a general rule, it is better not to decant too much in advance of serving it, even if some old vintages benefit from some time to breathe, for the risk of making a mistake is too great."