“HRH Prince Christian has a birthday and is 13 years old,” the palace wrote alongside the image, which was shared to Instagram.
“Here the Prince is photographed by one of his classmates in a day of everyday life as a school boy in the 7th grade.”
It was 2011 when Prince Christian was pictured attending school for the very first time – and it is clear to see in the picture just how much he has grown.
The simplicity of the photo was a hit amongst fans of the royal family with commenters calling him a ‘handsome prince”.
Australian-born Princess Mary and her husband Prince Frederik of Denmark have four children together.
As well has 13-year-old Christian they have Princess Isabella, 11, and twins Princess Josephine and Prince Vincent, seven.