Read a label
The label on a bottle of Cognac should accurately reflect the origin and the age of the youngest eau-de-vie used in its blend.
To be sold to the public, a Cognac must have been aged in oak cask for at least two years counting from the end of the distillation period, that is April 1st of the year following the harvest (compte 2). Once bottled, a Cognac, unlike wine, doesn't evolve anymore. Therefore it retains the same age indefinitely.
The most widely used designations are :
V.S (Very Special) or *** (3 star)... Compte 2: Cognacs whose youngest eau-de-vie is at least two years old.
V.S.O.P (Very Superior Old Pale), Reserve... Compte 4: Cognacs whose youngest eau-de-vie is at least four years old.
Napoléon, X.O (Extra Old), Hors d'âge... Compte 6: Cognacs whose youngest eau-de-vie is at least six years old.
Generally speaking, Cognac Master Blenders use eaux-de-vie that are much older than the minimum requirement for their blends. In fact, the most prestigious designations may have aged for dozens of years in oak casks before being presented to the public.
See the animation (in French) « how to read a label »
(animation of BNIC Bureau National Interprofessionnel du Cognac)
Learn more : Understanding a Cognac label (pdf from BNIC)