Decanting wine does typically two things to your wine: it separates the wine from any sediment that may have formed and also aerates the wine at the same time. While some people feel that extra boost of oxygen can open up a wine and give it extra life, while a few others feel it makes a wine fade faster, and that swirling wine in a glass, for 10 seconds or longer, is sufficient aeration. This goes for both white and red wines old or young.
A particularly fragile or fairly old wine should only be decanted for about 30 minutes or so before you’re going to drink it. However, a younger wine, more vigorous, full-bodied wine can be decanted as much as an hour or more before serving. I’ve had wines that were decanted for hours and even days that were still showing beautifully, but these experiments can be risky and I don’t recommend it without some thought.
One more tip about handling an older wine—set it upright for at least a day or more before decanting, so the sediment can settle back down to the bottom of the bottle and make it easier to separate while decanting.
While wines that are not too old are decanted by far over the older wines. So the worry about sediment isn’t a concern. If you are at a restaurant that has a sommelier and you want to decant a 4-5 year old Chardonnay don’t expect him/her to be shocked that you want to decant a white wine. This is good and has been going on for many years amongst those that are into wines. Try it for yourself.