The age of a whisky, specifically the amount of time a whisky spends in a cask after distilling (a.k.a maturation), has a huge effect on whisky, but does that magic number dictate the quality of the end product? No, it does not. It certainly drives the price up, and is extremely effective marketing. This is due to the general perception out there in the world, that if a whisky is old, it must be good. But this is definitely not the case, and really there is no magic number.
How does age affect a whisky?
This is a bit of a 'how long is a piece of string?' question, as alluded to above. In general terms, maturation in casks will gradually mellow the 'new-make spirit' (malt spirit fresh out of the stills which cannot be called whisky), blending and balancing the flavours, imparting flavours through both the oak wood in the cask (in the case of Scotch whisky), and also the remnants of whatever was in the cask previously, such as sherry or bourbon. So it stands to reason that more time in the cask will give more flavour to the whisky.
This is basically true, but it is not that simple. The flavours given can overpower or cancel others, for example a whisky can become overly-oaky in it's taste, particularly if the cask used was a first fill (has not previously been used to mature whisky), if it spends too much time in the cask. An over-aged / too old whisky can become fragile, it loses most of it's inherent flavours and quality, does not hold up to temperature variations, time in the glass, or water being added, as well as it could have, and becomes less complex and interesting than it may once have been. And, probably the biggest effect of a whisky being aged/matured for too long, is that it generally loses alcohol content (and whisky content in general) through evaporation, each and every year it is maturing. Although this can vary depending on local climate and specifically humidity, in some cases the alcohol content will increase!
At the other end of the scale, a whisky which has not had enough time to mature in the cask / is too young, will often be 'hot', in terms of alcohol, which will give you that trade mark 'burn' in the throat (and make you cough if you're 'inexperienced'), make your tongue go numb, and will singe your nasal hairs when you nose the glass. The flavours will be not be balanced, and the whisky will be very robust and overpowering.
It is important to note, that the Scotch Whisky Association (SWA) requires a minimum of 3 years maturation in oak casks for the spirit to be called Scotch Whisky. There is no limit to how many different casks can be used, what the previous contents was, or what size the casks are, for that maturation period. There is no maximum maturation period, either.
There are many different size casks available to a whisky distillery, and many different 'previous owners' to choose from, as in what liquid the cask held previously, and a cask can (and usually is) re-used or recycled after use. All of these factors (and many more) have a huge effect on the finished product. But these are subjects for another day.
One more major effect of cask maturation, on peated whisky, is that the peaty, smoky flavours diminish over time, lost to evaporation (called the 'angel's share'), wood contact and general mellowing. So, in my opinion, be wary of any peated or heavily-peated whisky which is over 18 years of age. It may still be a great whisky, but the level of peat & smoke flavour present may not be what you were hoping for.
As an example, Bruichladdich Octomore, the most heavily peated whisky in the world, is usually aged for only 5 years, allowing the whisky to mellow (slightly), while still retaining the maximum peaty, smoky kick. At the other end of the scale, they have recently released a 10 year old version, which is still bottled at cask strength, but is considerably lower in alcohol, peaty-ness and smoky-ness. It's still good, but different, and much more expensive at around twice the price.
To combat this effect of maturation, distilleries will often bottle their older whiskies at higher alcoholic strength or proof, and also to give better value to the customer, justifying the increase in price (in some cases). So all is not lost, but this is also a subject for another day.
The No Age Statement 'NAS' uprising
As the popularity of a whisky increases, so does the pressure on the distilleries to get the stuff on the shelves as fast as possible. If it isn't there, they can't sell it. So, rather than having a lot of stock sitting in their warehouses, maturing peacefully, many distillers will blend (still a single malt though, remember) some younger spirit with some older, and either state the age of the younger spirit (as required by the SWA rules), or simply leave that part out, and not tell you how old the whisky is, because they don't want this to influence your decision. This is known as a No Age Statement, or NAS, bottling.
This does not make it a bad whisky, or a good whisky, you simply do not know how old the contents of the bottle are. This is not a very popular thing in the whisky world, as you can't form an opinion based on the age of the product, and also cannot use that to gauge the value for money you are / are not getting, which makes it more difficult to make an informed decision, come buying time. Naturally the assumption is that if they're not telling you how old it is, some or all of it must be young. And to be fair, this is often correct!
They recently released Talisker 'Storm' and 'Dark Storm', neither of which have an age statement. The Storm and Dark Storm are bottled at the same stregnth as the standard 10 year old (45.8%), and are at a similar price as well. So naturally one would assume they are a younger product than the 10yo bottling, or at least contain some younger whisky. This one fact does not make them better or worse than the 10yo, but depending on your perspective, does perhaps give you less value for money, as you are getting at least some less mature whisky for the same amount of money, at the same alcohol percentage.
So what's the golden rule, then?
Well, there isn't one, sorry! While age / maturation has a big effect on the whisky, it is only one of many, many factors which the distiller, and their customer, must take into account. That number on the label should not be the deciding factor in your purchasing decision, because there are many other factors of equal importance.
If you can't try before you buy, then read reviews online, ask your friends or independent experts, and make as informed a decision as you can. Happy hunting!