Prince George’s teenage years have officially begun with his 13th birthday marked in two very different ways.

In what has become tradition for his parents, the Prince and Princess of Wales, a new official portrait was released of the future King on his big day, showing him posing for the camera in a suit following Trooping the Colour last month.

While hours later, William and Kate gave a glimpse at their eldest son’s relaxed side by sharing a short behind-the-scenes clip showing a casual George on holiday grinning in sunglasses, playing cricket on the beach and petting the family dog.

Both were posted on social media with sweet messages – but notable in the fact both referred to the heir to the throne simply as ‘George’, omitting his royal title. And in birthday posts for their other children, Princess Charlotte and Prince Louis, they too were referenced by their first names only.

And while it might be a small detail, royal expert Jennie Bond believes it speaks volumes – especially when it comes to his father Prince William and his vision for a modern, more relaxed and less stuffy monarchy.

The former BBC royal correspondent told the Mirror: “I love the fact that they refer to him – and their other children – simply by their Christian names, without the formality or pomposity of using their titles. It sets a new tone, a tone that William has spoken about: being royal with a small ‘r’.”

This year’s photo of George to mark his milestone birthday is a marked difference from last year’s, which showed him in country casuals in Norfolk.

The new snap, showing him suited and booted, is also in contrast to snaps that have been released of Charlotte and Louis, which have also shown them in more relaxed settings.

And Jennie believes this slight shift for George could be for a particular reason. She explained: “For William and Catherine, it is a way of protecting their children, and it is working well. That’s partly because the pictures are clearly quite relaxed and informal.

“And this year we got a bit of both: the fun video of George on holiday with his siblings, and the much smarter one in his suit (happily without his tie!).

“These pictures will be part of the history books, so I suppose it’s important to start seeing this lovely young boy as a future King. And seeing him in his suit helps mark his transition from child to teenager.”

George made his first public appearance at just a few hours old when he was cradled in his mum Kate’s arms on the steps of the private Lindo Wing of St Mary’s Hospital in Paddington, London.

No longer the tiny baby, the youngster is now as tall as his mum, with his 13th birthday coming at a pivotal time in his life. In September, he will follow in his father William’s footsteps by starting at the £63,000-a-year Eton College, where he is expected to board full-time.

It also comes as the young prince has been making more and more public appearances alongside his parents at the likes of Trooping the Colour – while last month he carried out his first official joint engagement with his mum as they visited the Battle of Britain Memorial Flight at RAF Coningsby.

But with his parents still determined to let all of their children have as normal a childhood as possible, more public duties at this stage seem unlikely.

And for Jennie, this means the birthday photo tradition for George seems likely to continue into the future.

She added: “I suspect this tradition will continue because it’s part of a bargain whereby one or two excellent images are released to keep us all informed about how George is doing – and then he is left alone.

“Over the years, the photos have shown a young boy smiling and laughing, happy and confident – far better pictures than a bank of press photographers could have achieved. And so it has been a highly successful tactic.”