The 'age' of whisky depends upon how long it matures in oak casks. If you bought a '5 year old' whisky 30 years ago, it's still regarded as 5 years old, not 35 years old (because the flavour won't have improved during the storage period).
So your whisky won't be worth any more than the same number of bottles of a similar whisky purchased today. It's illegal (under most circumstances) to sell spirits without a licence. If you'd got a genuinely rare product, you'd be able to sell it through one of the specialist auction companies which trade in wines and spirits but, with a 'bog standard' product (irrespective of how long it's been sitting in your loft),your best bet is to sell it to a mate. (I'm assuming that you don't want to drink it!) . Given that a basic supermarket blended whisky is probably around �8 per bottle, you'd need to sell a similar whisky for slightly less to get a buyer interested. However, if it's a decent single malt, a supermarket might charge between �15 and �25 per bottle, so you could ask more for that type of whisky.
Of course, if you've got a really expensive single malt, such as Imperial Tribute (which currently retails around �135 per bottle), it might be worth entering it into auction, especially if it's got a label design that's no longer in use, as it might be of special interest to a collector.
Chris