Time changes a wine in the bottle. In a red, the tannins slowly bind together and settle as sediment, so the wine grows softer and paler, its colour turning orange at the rim and then brown. The fruit of youth gives way to a more layered bouquet, and the finish grows longer. A wine reaches a peak and then declines, and that moment varies from bottle to bottle, even within the same wine.

Only some wines are built to last this long. In reds, tannin and acidity carry them: Cabernet Sauvignon in Bordeaux through its tannin, Pinot Noir in Burgundy through its acidity. In whites, acidity plays the part of tannin, which is why Riesling ages so well, and sugar helps too, as in Sauternes. Most Champagne is sold ready to drink, but vintage Champagne can age on its acidity. Vintage Port is made to age in bottle, while most wood-aged Port is ready when it is sold.

Before buying an old bottle, look at its fill level and its cork. Auction houses describe the level in a Bordeaux bottle by where the wine stands: in the neck, top shoulder, high shoulder, mid shoulder or low shoulder. Top shoulder is normal for a wine over fifteen years old. Mid shoulder can be normal at forty years but carries some risk, and low shoulder is a real risk. A Burgundy bottle has no clear shoulder, so its level is given in centimetres below the cork: four is normal after twenty years, five is not unusual after thirty. Catalogues also note a cork that has sunk or risen in the neck, called depressed or protruding, and signs of seepage, where wine has slowly escaped past the cork.

For a wider choice of wines fifteen years and older, see the collector wines.