A common belief, often held by even wine lovers, is that all wine can benefit from aging after purchase. Not only is this not true, but it can lead to terrible disappointment. The fact is, the vast majority of wines will not benefit at all from aging, but will actually decline in quality with further aging. It's likely the case that many more wines have been consumed past their primes than consumed too young.
Most wines are released when they are ready to be consumed – they are intended for, and even designed for, immediate enjoyment. Experts agree that more than 90% of wines were designed for immediate consumption. The exceptions are very fine, complex wines, that will mellow and mature with aging – likewise, they are designed for aging over time.
So, how do you know whether a wine will benefit, or at least not suffer, from further aging? Generally speaking, big, tannic red wines will mellow with age, and actually need some aging in order to mature past the bitterness they have in youth. Less tannic reds have no real need for further aging, and the benefit of aging is less apparent. Most whites will not benefit from aging, with fewer exceptions than reds. Those that do, begin with a high level of acidity.
In general, Cabernet Sauvignon is most likely to benefit from aging, along with other reds, to a lesser extent, including Hermitage, Pinot Noir, Barolo, Barbaresco, and some Shiraz. In white wines, White Burgundy and Rieslings tend to improve with age, along with White Bordeaux and Gewurztraminer.
Surprisingly, price is also a key indicator of likely improvement with age – expensive wines are the most likely candidates to benefit from aging. A rough rule of thumb is that the more expensive a wine is, the more aging is appropriate. For example, a mid-priced Cabernet Sauvignon, in the $12-$25 range, is drinkable immediately, but could benefit from 5-6 additional years of aging. Less expensive bottles are designed for immediate consumption, and won't benefit much from aging. More expensive bottles are likely to improve with 7+ years of aging, while very special bottles can improve for decades.
So, unless you invest in premier wines, you likely do not need to age your new purchases at all. Regardless of whether a bottle of wine will benefit from aging, care should be taken to store it properly – in a consistently cool, dark, relatively humid environment, so that it can be enjoyed to its highest potential when you're ready to enjoy it.