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Whiskey most definitely is aged in oak casks, for quite a long time at that. Some distillers use fresh casks while others use casks that had been previously used for sherry. Some may use a sequence of casks even, or may have different types/lines that require certain types of casks. I know the scotch [theglenlivet.com] I drink has several different vintages. They age for a various number of years, again for the Glenlivet, that can be 12, 15, 16, 18, 21 years or more. The difference between each vintage is noticeable, primarily in the smoothness and variety in tastes.

My family has been distilling for generations, and finding ways to "age" things has been around forever. "Aging" is a nice ancient technique to make up for not having advanced technology at their disposal.

As far as cask aging, which I saw a few posts on, it has nothing to do with evaporating heavier alcohols (where would they go, and, there's is only one alcohol, ethanol).

Many distilleries use white oak casks, which receive a 1200 degree firing of the interior to charcoal the insides before the product is added. This is one of the causes of the "brown" color of those liquors that use this method as well as the "smoke" flavor, and is used to basically create an activated charcoal filter that the product lives in for "years".

When the barrel is fired (and then extinguished with steam blasted in) the char has all these nice little pathways and tiny cracks whose job is to grab all these taste screwing large molecules that give a harsh taste to the product. Just like activated charcoal is used in a water filter for drinking water, the same technique mellows the flavor of the liquor. The "aging" is the act of, as summers and winters went by, the casks would "breath" due to the contraction and expansion of the cask due to temperature variation which would circulate the product in a fashion to get the filtering going with pressure changes. The more that occurs, the more it is filtered, the cleaner the taste.

These molecules that we're trying to get rid of are some of the products of the distillation. When you distill your mash or beer, you have a variety of products separated from the water, the heads (where the majority of your flavors come from), the ethanol, and the tails (fuseoils, which are the disgusting taste). When distilling you carefully test the product coming out and separate it into the various products (if using reflux distillation with plates). The heads are high volatility and the tails are high weight. The tails are smelly and screw up your taste so you have to be careful distilling to get the correct balance of the middle of the distillate, but not losing the flavoring agents of the heads or tails from the heart of the product.

If you distill and filter over and over, you get "pure" ethanol or the basis of vodka. The ethanol purity is only about 95.6% as the distillate reaches azetrope, meaning you can't really separate it from what it's being boiled off of. There are methods to get beyond this such as vaccuum distillation to separate your distillates or post distillation methods (steam blasting through oeatmeal for example or even using gasoline) to use adsorption to remove the last remaining bits of stuff you don't want. Of course, if you leave a bottle of 100% ethanol out, it'll go back to 95.6% as it exchanges water from the air.

Aging has no real meaning these days. The point of aging is to use activated charcoal to remove things you don't want. You don't want the big molecules that cause bad taste, you want it filtered from the product. You do want to keep some though, which are in the "heads" because they have the specific flavors you want to distinguish your liquor. You can't use a perfectly pure vodka base, because then you've gotten rid of all those

Today, as part of your distillation process, after the product has gone through fractional (reflux) distillation through your column, it is common to "force" it through several packs of activated charcoal, in order to quick filter it. This is used to get the purest base ethanol in vodka creation, and why you see different marketing of "triple filtered" or "6 filtered" vodka, claiming how many filter processes it goes through to remove taste impurities.

Whisky ages by evaporating bad alcohols while retaining tasty ones.

This statement is nonsensical. Whisky, and any other alcoholic drink for that matter, has one and only one alcohol, ethanol, C2H5OH. At least, it better, since any other form of alcohol is quite poisonous.