The age of a whisky is determined by the number of years it spends in oak casks. Also, the whisky gets its age based on the youngest whisky in the barrel. So for eg. If a whisky label says 15 yrs old, the youngest whisky is 15 yrs old, but it could also have whiskies which are 16, 17, or 20 yrs old too. Isn’t that fascinating?