Scotch whisky is matured in oak casks, in Scotland, for a minimum of three years. There is no maxiumum amount of time it can be kept in cask, and some whiskies have been bottled at fifty years and older. Whisky continues to mature in cask, but once bottled the whisky does not improve.
How old a whisky should be is a matter of personal preference and taste. Because Scotch is matured in second-hand oak barrels it takes longer to mature than whiskies that use new oak casks. Whisky also matures more slowly in the cool climatic conditions of Scotland than in many other whisky-distilling countries where summer temperatures and humidity are high.
Time spent in cask is usually a good and reliable indicator of quality, but very important too is the quality of the cask and the type of wood selected for the maturation period. Casks that have previously been used to mature sherry or bourbon whiskey give Scotch whisky distinctly different flavours and the older a cask is the slower it will mature a spirit. Ten years of maturation in a good quality cask is preferable to longer in a poor one.