Which got us wondering, how did the lineup celebrate their own milestone?
His big day was in 2015 and he marked it in style with a huge bash in Las Vegas. Fresh from filming his Comedy Central Roast, he partied in Sin City with the likes of Kris Jenner and Pretty Little Liars' Ashley Benson, while DJ Martin Garrix provided the beats. It's rumoured he was paid $500,000 to appear at the Vegas nightclub. Not bad when you'd be out celebrating anyway, right?!
The 'Umbrella' singer was born 20 February, 1988, so she marked her 21st birthday in 2009. Rihanna reportedly went low-key and joined friends at the cinema to watch Tyler Perry's Madea Goes To Jail, before a private party with her nearest and dearest in the Hollywood Hills where her friends serenaded her.
Little Mix's Perrie Edwards
Perrie was treated like a princess by then-fiance Zayn Malik for her 21st in July 2014. Zayn helped organise a funfair at a park in North London near their home, with guests treated to bumper cars, a bouncy castle, a Ferris wheel and unlimited pick 'n' mix. Celebs at the event included her Little Mix bandmates.
Little Mix's Jesy Nelson, Jade Thirlwall and Leigh-Anne Pinnock
Jesy Nelson, Jade Thirlwall and Leigh-Anne Pinnock all marked their 21st birthdays with glamorous dinner and drinks in London… though Leigh-Anne's didn't have the most glam of endings when she was mooned on the street by a random man who dropped his trousers and flashed his hairy bum for the cameras.
It was just a few months after releasing breakthrough single 'RIP' in 2012 that Rita Ora marked her 21st birthday on a tour bus. She joined DJ Fresh at Alexandra Palace, where they sang 'Hot Right Now' for an audience of 4,000 and the audience sang 'Happy Birthday' to her. "The impact was crazy, the vibe was crazy."