Whisk[e]y varieties often have significant differences in how long they are aged. While there are always exceptions, Bourbon is often aged for 5-10 years. Scotch, on the other hand, is more commonly aged for 10-20 years. Why is this?
Aging or 'Finishing' is an extension of the maturation process, when the spirit is subsequently filled into empty casks that previously held other wines or spirits for a further relatively short period at the end of maturation.
The selection of casks can affect the character of the final whisky. Outside of the United States, the most common practice is to reuse casks that previously contained American whiskey, as US law requires several types of distilled spirits to be aged in new oak casks. To ensure continuity of supply of used oak casks some Scottish distilling groups own oak forests in the US and rent the new barrels to bourbon producers for first fill use. Bourbon casks impart a characteristic vanilla flavour to the whisky.
so really the longer you age the whiskey the more of the flavor you are going to capture form the cask
First, we have to understand why we age whiskey in the first place. It's primarily to extract compounds from the wood of the barrels (and any previous contents) so that they can add flavor and complexity to the final product.
From there, there are several factors that determine how long a typical aging period might be.
If the wood in the barrels is new, or has been previously used. Bourbon, which is typically aged as few as 4 years is aged in casks of new wood, meaning there are more compounds in the wood to be extracted, and they can be extracted more quickly. Scotch, on the other hand, which is often aged from 10-25 years, is aged in previously used barrels, so it takes longer to extract the desired level of flavoring compounds.
How much flavor you want the wood to add to the finished product. A Rye, aged for 10 years, is going to have a lot more oak than you would want in say, an Anejo Tequila, which will often be aged for just a year or two.
The type of wood can have an impact as well. Most whiskeys are aged in American or French oak, but if other woods are used, a tighter grain, or different wood chemistry can mean that you need more time in the barrel to the get the flavors that you want.
A true Scotch whisky has to be aged at least 5 years to qualify as whisky. Beyond that, the factors that change over time are:
The whisky takes on these from the wood, and from the previous contents. So for a really well rounded whisky, especially for a full bodies peaty, smoky whisky, that age is essential. That said, there are some amazing young whiskies, and some older whiskies that aren't as nice, so this comes down to taste,
In fact there is a current furore in the industry about No Age Statement whiskies, as various blends are being sold off young, potentially putting the longer term whiskies in jeopardy as volumes will be necessarily reduced in the long term, becuase of a drive for shorter term profits...
- The alcohol content decreases as the Angel's Share leaves the cask over the years.