The age statement on a whisky label tells you the age of the youngest whisky in the bottle. What does this mean? Put simply, a bottle of whisky may contain whiskies matured in a number of different casks for different periods of time. If the label says that the whisky is 15 Years Old (or Fifteen Years Old) then, although it may contain older whiskies, you can be certain that none of components are any younger than 15 years.
Once a whisky is bottled it ceases its maturation, unlike wine which continues to age in the bottle, so Fifteen year old whisky is frozen in time and will be considered 15 forever.