User blog comment:Hollyleafisawesome/Create a Cat Fur In The Little Woods!/@comment-11587616-20140212011156/@comment-5137480-20140214123552

An apprenticeship usually lasts for six moons or more (Except for Graystripe and Firestar, who became warriors at 9 moons, also if you were away like Squirrelflight or broke a bone like Crookedstar your apprenticeship is longer), thus, warriors are older than 12 moons. Cats become warrior apprentices after an Apprentice Ceremony, and full warriors after a Warrior Ceremony.

From Warriors Wiki~