An aged wine is one older than a winery’s currently released vintage, and which has been in the bottle for at least 1-2 years.
It’s not uncommon for a winery to allow its wines to age for 18 or even longer, to reach the right quality and be ready for release. This means a wine produced in 2017 might not be bottled and released by the winery until sometime in 2019. Even then, the wine is going to need time in the bottle to develop its own characteristics.
Most white wines age gracefully, but with a shorter lifespan than reds. This has to do with the chemical composition of white wines: Fewer tannins; Save for Chardonnay, most never see any oak; Lighter and more delicate flavors that may not improve with time; And so forth. White wines generally diminish in quality faster than reds.
Conversely, many red wines need a certain amount of ageing in the bottle for their flavors and qualities to emerge. Acids and volatile compounds interact with tannins and ultimately smooth out. Many red wines have a “sweet spot” of ageing at 4-6 years in the bottle. Some may need up to 8-10 years to really hit their max. After that, it may be a slow decline as they gracefully age.
When purchasing an aged wine, such as from a winery during a library sale or when getting a private tour of their cellar, your best bet is to ask the winemaker or winery staff for their recommendations. If you’re lucky, they may be pouring a selection of aged wines as part of the event, so you can get a first-hand taste of the product for yourself.
Generally speaking, if you’re thinking of buying aged wine, if it’s available in a magnum, chances are much better the wine is in better shape, than if just a 750mL bottle, as overall less oxidation has occurred throughout the wine.