A young wine is one that was recently bottled, while an aged wine may have set in its bottle for years.
Depending on the varietal, vintner, and growing conditions for the grapes fermented, a wine may be drinkable when still young, or may require additional aging prior to opening.
Young wines usually show their full aromas and flavors immediately after opening. After prolonged exposure to the air, young wines may “fizzle out”, lose their flavors, become flat and sour.
On the other hand, an aged wine may require ten or fifteen minutes of decanting to properly “breathe” and expose its hidden layers of flavors and aromas. Additional air-time will not adversely affect the wine.
Chemistry helps explain why aging a wine may enhance or inhibit its qualities. The process of bottle aging allows a wine to sit in a stationary position for a long period of time, its chemicals slowly mixing with minimal amounts of oxygen that manage to pass through the cork. For this reason, bottles sealed with Stelvin caps or rubber corks may not age in the same way as a bottle aged with a traditional wooden cork.
For red wines, bottle aging can allow some volatile compounds to lose their punch, and produce a subtler, complex set of flavors in the wine. These compounds – tannins, phenols, glycerols, and sugars, present in higher concentrations in red wine, may benefit from moderate to long-term aging.
By the same token, aging a white wine will likely not benefit its qualities at all. Its lighter concentration of chemicals could be overpowered by lengthy aging, resulting in a flat, dull wine when opened.
Regardless of varietal, all wines have a cut-off point, after which time additional aging will not benefit their qualities. The cut-off can be determined from vintage to vintage, winery to winery, and, if you are collecting wines in a cellar, is something to carefully track.