The overwhelming majority of wine made today is intended to be drunk within a year or two, and keeping it does nothing except make it worse. Ageing is a minority activity that applies to a minority of bottles, and knowing which is which saves both money and disappointment.
What lets a wine age
Three preservatives, in various combinations. Acidity is the most important, which is why German riesling at 8 percent alcohol outlives most heavy reds. Tannin is the second, which is why Barolo and classed-growth Bordeaux need a decade. Sugar is the third, which is why Sauternes, Tokaji and vintage port last for generations.
Alcohol also helps, which is why fortified wines are so durable. Fruit intensity does not. A soft, fruity, low-acid red with no tannin will not improve; it will simply fade, and its charm is the first thing to go.
What ageing actually does
Tannin polymerises: individual molecules link into longer chains that eventually fall out as sediment, so the wine feels softer and looks paler. Primary fruit aromas fade and are replaced by tertiary ones, leather, mushroom, tobacco, dried fig, forest floor. The wine becomes more complex and less obviously delicious, which is why some people who try a famous old bottle are disappointed.
Whites go the other way, deepening in colour from pale straw to gold while reds lighten from purple to brick and then to tawny at the rim.
The four conditions of storage
Temperature is the only one that really matters. Aim for 10 to 15 degrees Celsius, and prize stability over the exact number: a steady 18 degrees does far less harm than a cycle between 8 and 24, because repeated expansion and contraction pumps air past the cork.
Darkness matters for sparkling wine and light-coloured bottles. Ultraviolet light produces the fault known as lightstrike, which smells of wet wool and cooked cabbage, and it is the reason so many bottles are dark green.
Humidity around 60 to 70 percent keeps a cork from drying out. This is only relevant to cork-sealed bottles kept for years. Screwcapped wine does not care.
Lie cork-sealed bottles on their side so the wine keeps the cork swollen. Screwcapped and sparkling bottles can stand upright.
What to do without a cellar
An interior cupboard away from the kitchen is better than most people assume, because interior walls buffer temperature swings. The two places to avoid are the top of the fridge, which is warm and vibrates, and any spot near a radiator, an oven or a sunlit window.
For anything beyond a few years, a small wine fridge set to 12 degrees is the cheapest real solution. It is bought for temperature stability rather than for cold.
An opened bottle
Recork it and refrigerate it, whites and reds alike. Cold slows oxidation, and a red can be brought back to temperature in twenty minutes. Most wine is noticeably tired after two days and drinkable for three or four.
A vacuum pump helps a little. Half bottles help more: decanting the remainder into a smaller, completely full bottle removes the air entirely and is the most effective home method there is. Fortified wines are the exception and keep for weeks, and Madeira keeps for months.