Wines that have thrown a deposit must be decanted, and the only questions are about practicalities. The debate over whether or not to decant other wines centres on the age of the wine and the time it is allowed to spend in the decanter. Enthusiasts for decanting say that young wines can be softened, rounded and have their taste generally improved by decanting an hour, even several hours, before they are drunk.
The counter-argument is that a lengthy period can allow the wine to lose crispness and vitality. Older wines may be freshened by decanting — but they may lose some of their precious aromas in the process.
Few doubt that the aeration caused as one pours the wine into a decanter is beneficial. Even white wines, especially mature ones, gain from this. The air releases aromas and tastes which are dormant in the wine. The question is when to do this: just before serving, or earlier.
Another argument for decanting is that it allows young wines to "age" quickly; to mimic the effect of bottle-ageing. This is more controversial: no-one is quite sure how and why wines age in bottle; the chemical reaction involved is complex and not fully understood.
Beware of decanting too soon: wines that are mature, or even past their peak, will fade quickly in the decanter. It is always best to allow too little rather than too much time. The beneficial effects of aeration will continue when the wine is poured from decanter to glass, and wines can be aerated by gentle swirling in the glass to speed up the "decanter effect"