What are the main qualitative and quantitative differences between 18 and 12 year old Scotches?

Question

It's established in the Scotch world that the age of a whisky pushes it's price up. A MacAllan or Glenmorangie 18 will fetch a much higher price than their respective 12 year old versions.

Having tried a small number of 18 year olds, I've found them highly variable in quality, just as you would with those in the 12 category, but those which were of higher quality seemed to be more mellow and subtle than their twelve year old counter-parts. But that's only my understanding from a very small sample size.

So the questions are:

• What does aging a Scotch do to the whisky chemically?
• What properties do 18 year old Scotches tend to share for tasters?
• Are 18 year old Scotches ever crafted with special care, to make them superior aside from the aging process?

Answer 1

Right now I can think about some characteristics of a Sotch, that depend partly on the age:

1. Flavours: The longer a whisky matures in a cask, the stronger is the influence of the cask on the flavours. So the character of a very young whisky largely comes from the distillery. That often means some fresh, fruity flavours, with a significant alcoholic taste. The character of an older whisky is largely defined by the cask(s). That often means some sort of wooden and softer flavours, besides other flavours that depend on the concrete cask.
2. Color: Assuming the same cask, an 18 years old whisky is always darker than a 12 years old one, because the former had more time to absorb the color from the cask. (Ignoring the fact that many distilleries add caramel colouring before the whisky is bottled)
3. Alcohol and volume: Over the years, the whisky constantly evaporates through the cask. Therefore, the amount of whisky in the cask is constantly decreasing (that's one reason why old whisky tends to be more expensive than young whisky). As alcohol evaporates easier than water, the alcohol concentration is decreasing as well.

I'm sure that a distillery doesn't use the same distillate for all products. But I have no idea if that means, that the distillate that is designated for an 18 years old whisky is crafted with more care than the one that's designated for a 12 years old whisky. Only differently, I guess.