The old adage "all wine improves with age" is only partially true.
Only a few varieties of wines actually get significantly better with age. Only about 10% of red wines and 5% of white wines taste better after aging five years as opposed to aging one year.
What does aging wine mean today
Most wines these days are specially crafted to be enjoyed shortly after bottling. According to Master of Wine Jancis Robinson, it is more typical now that wine is being consumed past its prime rather than while it's too young.
In general, many wines start to lose a majority of their fruitiness and appeal after being bottled for only six months. The way it happens is wines with a lower pH, such as Pinot Noir, have the greatest ability to get better with age. A lower pH is usually achieved in red wine by the addition of tannins, thus increasing the amount of phenolics in the wine. White wines that do well with age are those that have a high acidity level. The phenols and acid found in these wines act as a preservative and start to break down and mellow out over time.
Today, many wine makers are starting to bottle wines when they feel the wine is at the peak of flavor. This is in part due to the fact that wine makers are aware that consumers have become a 'microwave society' - meaning consumers don't want to buy a bottle of wine and have to wait to consume it until it ages in the cellar. We want to buy a bottle of wine and uncork it that night.
When white wine is made, the producer tried to keep the skin contact to a minimum. Having contact produces phenols and tannins in the wine and keeping the contact down means the wine will have significantly less phenolic compounds. The only time these phenols are introduced is when the wine is fermented in oak barrels or is left to age in the oak barrels. The contact with the wood over an extended period of time will impart a small amount of phenols into the wine, but not enough to make aging after the wine is bottled worthwhile. The same goes for rose wines, thus reducing their aging potential.
Unlike white wines, reds have a very high percentage of skin contact when making the wine and are usually filled with bitter tannins. As the red wine ages, the harsh taste of tannin slowly gives way to a softer, more full-bodied wine. This can be noted in the color change, from a deep red, almost black, to a lighter red as it ages. Once the wine is past its prime, the color turns to a brownish hue.
As the tannins start to give up some of their bitterness, sediment starts to form on the bottom of the bottle. The presence of this sediment usually indicates a mature red wine, but is separated out by decanting to avoid the bitter taste. Vintage Ports and other bottle-aged Ports and even some Sherries will benefit from some additional aging, but many other red wines start to diminish after three to five years.
As wines start to age, their floral bouquets will start to become more prominent, but today most of this aging is done before the wines are ever bottled, thus allowing us to go to the store, pick up a bottle and enjoy it at its peak that evening.
Most fine red and white wines improve with age. Not so in the case of light reds, roses and most whites. Aging wine results in a smoother, less tannic drink. Flavors develop and the wine gains complexity.