Well, im a total noob, but surely its common sense that it depends on more of the content than the age of the domain.
e.g. lets say Domain1.co.uk has been registered for 5 years, and in that time has had little done with it, sure the domain may still be a little valuable, but then if Domain2.co.uk has only been registered for 6 months, but in that time has became a reasonably popular chat site or forum for food critics, has about 3000 members and daily unique visits of 500+ visitors, then Domain2.co.uk would likely sell for a huge amount more than the first.
I like the plot of land scenario that murray mentioned, sure if it's good enough it will be of potential to investors and developers and age may help a little with that, but in the end, it's the development that will really make the huge profit when it comes down to re-sale.