You may have seen the Instagram trend that's been doing the rounds this week – people sharing photos of themselves aged 21.
"Everyone tap in. Let's see you at 21" posts have been popping up on feeds. We know that some of the royals have their own personal – and very private – Instagram accounts, like Princess Beatrice and Zara Tindall. Have they been caught up in the craze?
As we'll never be privy to that information, we've taken a trip down memory lane and dug out some old photos of some of our favourite royals aged 21.
Here's Prince William running along a beach near his university home in Scotland in 2003. He was in his second year at the University of St Andrews.
Dr Brian Lang, the Principal and Vice Chancellor of the university during William's time, told HELLO! how the university struck up a deal with the British press to ensure that William would be left alone to enjoy his student days.
"Andrew Neil, the Lord Rector of the university, being a very experienced newspaper man, helped us negotiate a deal with the British press, whereupon they would leave William alone in return for a photo opportunity once a semester. And on the whole, that worked.
"And, one thing that I did as soon as we knew that William was going to be a student, was to warn the student body that we expected them to respect privacy of information. So, I made it quite clear to the students that we would take a very, very dim view of any inappropriate information being passed on. Not just about William, but about any other student."
As for William's actual birthday party, he had an Out of Africa-themed bash at Windsor Castle. There was even a vodka luge in the shape of an elephant, which was said to have greatly amused the late Queen Elizabeth II.
Disclaimer: this photo is from the Princess of Wales when she was 19 on her gap year in Chile. Prince William's future wife spent ten weeks in Patagonia, doing wilderness trekking and anaylsing marine life.
Prince Harry had just started training at the Royal Military Academy Sandhurst when he turned 21 in September 2005. He took a break from his officer duties to give an interview to the British press, where he spoke about his then-girlfriend Chelsy Davy, his stepmother Camilla (who had just married Prince Charles five months prior), and his relationship with his brother William.
On the latter, Harry said: "It's amazing how close we've become since my mother died. Obviously, we were close, but he is the one person on this earth I can actually talk to about anything, and we understand each other and we give each other support and everything is fine."
Here's Charles at 21, on holiday in Barbados in 1970.
His investiture ceremony to become the Prince of Wales took place four months before his milestone birthday. His mother the late Queen officially presented him with the title at a ceremony in north Wales.
Here's Anne looking as cool as ever aged 21, proving just how much of a style icon she was (and still is!) back in the day.
Princess Anne's daughter has been a regular at the races and virtually any equestrian event since she was a young girl.
Here's a fresh-faced Zara looking all glammed up for Ladies Day at Royal Ascot in 2002.
All about the Tindalls
Fashion has long been a passion for Princess Beatrice. Here she is aged 21 about to take her front row seat at the Issa London show for London Fashion Week in 2010.
You may recognise the brand – it was the label that Princess Kate wore for her engagement announcement to Prince William. Cue: the Kate Effect.
Here's Princess Eugenie enjoying a night on the town in 2011. She was attending a charity event in London to celebrate Freddie Mercury's 65th birthday.
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