The Danish Royal Family's official Instagram account shared two images of the 16-year-old to mark the monumental occasion. The two pictures, taken by photographer Steen Brogaard, were snapped at Herlufsholm boarding school, south of Copenhagen.
The royal had just begun high school at the prestigious boarding school this September.
The caption reads: “His Royal Highness Prince Christian has a birthday and turns 16 today.”
“The Prince celebrates his day with the family, and on the occasion of the birthday, the Crown Prince and Crown Princess have the pleasure of sharing new images of His Royal Highness.”
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Instagram user, Maxirose07, commented: “The Image of Mary!”
Instagram user, Abi Jouni, gushed: “Such a handsome young boy, looks like his beautiful mumma.”
“Has his mother’s smile!” Instagram user, Rebecca Kate Mahomet, complemented.
Another user, Keila, wrote: “Happy Birthday Christian!”
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Another user, Tony, was shocked at the Prince's age.
“I can’t believe Mary’s little boy is 16!! Where have the years gone?” they wrote.
Crown Prince Frederik and Crown Princess Mary have four children together - Prince Christian, Princess Isabella, Prince Vincent and Princess Josephine.
Christian is the second in line to the Danish throne after his father Crown Prince Frederik.
The Prince was born in 2005 in Rigshospitalet, Copenhagen's University Hospital, as the first Danish royal heir born since his father Crown Prince Frederik in 1968.