Katy Perry has shocked fans by posting a throwback to her younger days on Instagram.The Dark Horse and California Gurls singer posted the photograph, which shows her as a fresh-faced 21-year-old ready to party at a roller skating rink, to her socials on Sunday in order to celebrate American Idol's 21st birthday.Find out why Katy believes this season of American Idol is the best yet in the video below.WATCH: Katy Perry introduces American Idol season 21Fans of the Teenage Dream singer songwriter rushed to comment on Katy's almost unrecognisable appearance in the photo from her early adult life."Fetus Katy!" one joked, surprised at the throwback picture.DISCOVER: Katy Perry, Lionel Richie, Ryan Seacrest's and more American Idol stars' stunning homes"You looked so cute at 21!" another added.Meanwhile a third person commented: "Always been gorgeous."A post shared by KATY PERRY (@katyperry)Katy's Instagram throwbackSeveral fans also left red heart emojis and teary-eyed faces in reply to the cute photo.Katy captioned her post: "[American Idol] is celebrating its 21st bday this year.