Think about it......older whisky just means whisky that has spent more time in a wooden cask.
Most of us have probably had the unpleasant experience of tasting a whisky that has spent too long in the wood. They're horribly oaky, dry & astringent, the wood has killed any of the complexity, the nose is usually flat, the delicate spices and floral notes are gone, and the whisky is quite simply tired and lifeless.
Once you go beyond 20 years, it is a very rare cask indeed that will continue to improve its contents rather than start to deteriorate them. And it is this rarity that reflects (and justifies) the price tag.
It's true that some of the most sensational whiskies I've ever tasted were in the 21-35yo range. It's also true to say that some of the worst I've tasted were in this same age range.
And I could say exactly the same for some whiskies I've had in their teens. A couple of 15yo to 18yo malts have been sublime, but I've also had a few that were just awful.
But even "youth" is no real indication. Whilst I doubt any 3 to 5yo malt would be worth getting excited about, one of the best Islay's I've ever had was a 9yo Caol Ila from the SMWS.
So what's my point? I simply conclude that age is absolutely
no indicator as to whether a whisky will be good, bad, or ordinary. As Jeroen said, increasing age simply means a more expensive price tag, but I've paid good dollars for older bottlings that were simply a disappointment to drink.
For the distilleries that feature quite a spread of ages in the line-ups, I (almost) invariably find that the most complex and interesting drams are the ones in the middle.
For example, with the Glenfarclas 10, 12, 15, 17, and 21 year olds, I believe the peak is at 15 years. (However, I have a private bottling of a single cask Glenfarclas 30yo which is just spectacular, but obviously this is a unique and special cask!)
With the Macallan 10, 12, 15, 18, and 25 yo, I reckon the 18yo is the most consistent and complete bottling. (I do miss the 15yo these days).
Bowmore Legend (NAS), 12, 17, and 25? Well, the 12yo and 25yo are two very different malts, but I enjoy them equally on their own merits. And I know which one I can afford to buy!!
Glenfiddich is a rare malt that seems to improve with age. The older bottlings (particularly the new-ish 30yo) are vastly better than the standard 12yo.
I could go on with more examples, but I'm probably boring you now, and I'm sure you see my point!