Aging

That's kind of like asking "how long is a piece of string?"

It varies, depending on the type and style of wine. Beaujolais isn't aged at all; typically, it's consumed within a year of harvest, at the outside. Generally speaking, red wines are aged longer than whites, but there are exceptions: it's not a bit unusual for a Tokaji to age 20 years or more.

Additional aging adds flavor and aroma. The alcohol reacts with carboxylic acids in the wine (chiefly tartaric, malic, and citric acids) to form esters; esters are the chemical compounds responsible for the flavors and aromas of most foods, including wines.

It's a balancing act: the longer the wine is aged, the greater the opportunity for these reactions to take place, and hence the more complex the flavor and aroma become -- but also the greater the opportunity for the wine to become oxidized. If you intend to age your wines for more than a couple of years, I recommend using #9 corks in the longest size you can find (typically 1-3/4") of the best quality you can afford. And make sure you have adequate sulfite levels in your wine; that's critical to prevent oxidation.