Age in cask.
To find a whisky which was distilled in 1981 and bottled this year would be a 29 year old. Distilleries would rather wait until January onwards then bottle it as a 30 year old.
Due to legal restictions with "Scotch" there is no 30yo from 1981.
You may find 25 yo which would have been bottled in 2006 but it's not the same as a 30yo on a 30th birthday.