Different malts mature at different rates. Lowlands mature quicker, for example - although, interestingly, they still hold their character well after even 30 years in the cask (vide: some of the old G&M bottlings of Bladnoch, St Magdalene and Rosebank). Many Highlands and Islays need much longer. Springbank is fabulous at 15 years but celestial at 30 years. Same with old Ardbeg and Laphroaig (although they loose some of their rougher Islay character). Old Talisker can be somewhat sulphury but is nevertheless magnificent.

The decision by a proprietor to bottle at a certain age is informed by the distiller's view of when the whisky peaks, as well as stock levels and economics.

Charlie