Wine, like any fresh food, changes with time. But whereas most consumables deteriorate from the moment we buy them, wine is one of the very few things we buy that has the capacity to develop, change for the better.
Perhaps the top 10 per cent of all reds and 5 per cent of all whites (it must be noted that these are generous estimates) will be more pleasurable, complex and interesting to drink when they are five years old - than at one year old.
The top one per cent of all wine made has the ability to improve for a decade or two or, in some cases, even more. The great majority of all wine, however, will actually start to lose the fruit flavours that gives it youthful appeal within six-twelve months of being bottled.

 

The more fruit, acid and phenolics that go into a bottle of wine at the beginning, the more complex interactions there can be between all these compounds and the more rewarding it can be to age. This means that the less water there is in the grape, the more likely it is that the resulting wine will repay cellaring.
Tannins and colouring matter known as anthocyanins are the most obvious types of phenolics and what preserves red wine as these interactions occur. These and other compounds continue to interact, forming bigger and bigger complex compounds which after a few years are too big to remain in solution and are precipitated as sediment.
So as good quality, concentrated red wine ages it becomes paler and softer to taste, while gaining considerably in the range of flavours it presents (which by now constitute a bouquet rather than simple aromas).
Any red wine with visible sediment (a fine deposit on the side or bottom of the bottle) is likely to have completed quite a bit of its development, ageing process.
Even less is known about how white wine ages, although acidity is thought to be the preservative white counterpart to tannin. Certainly, the longest-lived white wines are those with good extract but good acidity. The fact that white wines have far fewer phenolics explains why fewer of them can last as long in bottle (although botrytis can preserve some sweet white wines for decades and selective Tokaji wines even longer).