Many consumers believe that older is better when it comes to spirits. That's why you see those 40 year old Scotches selling for hundreds of dollars locked in glass display cases at liquor stores, and I would imagine that it's the reason that distilleries like Highland Park are expanding the range of bottlings to include older whiskies. In Highland Park's case, they've had a 12 year old and an 18 year old bottling for quite some time. The 15 year old bottling is relatively new, and I would imagine that one of the principal reasons that they introduced it was to appeal to consumers who think that older is better but don't want to spring for the 18 year old bottling. That's not to say that the 15 year old bottling isn't interesting in its own right or that the Highland Park people haven't striven to do different things with the 15 year old bottling from what they have done with the 12 year old (it does have a different aging program), just that I would be surprised if the driving force behind the introduction of the 15 year old bottling were not marketing concerns (namely, the need to get an older whisky out there to appeal to the "older is better" crowd).
Well, just as older is not necessarily better with wine, it's not necessarily better with spirits. The longer a spirit remains in barrel, the more it dries out and is dominated by the barrel's characteristics. Up to a certain point, that's desirable, but I generally would rather not suck on barrel staves when I drink a glass of whisky. To my palate, the 12 year old Highland Park is enjoyable than the 15 year old Highland Park -- richer, more complex, more interesting. I don't know whether the difference is due to the age of the whisky or the different aging program (12 year old uses more ex-Sherry butts than does the 15 year old), but I do like the 12 year old better. Which isn't to say that I don't enjoy the 15 year old very much or that I won't happily drink up the rest of the bottle.