The Duke of Cambridge turned 40 on June 21.
Prince William celebrated his 40th birthday on Tuesday with well wishes from members of the royal family. His father, Prince Charles, was among the first to share a message for his son on social media.
"Wishing The Duke of Cambridge a very happy 40th birthday!" a tweet on the official page for The Prince of Wales and The Duchess of Cornwall read.
The tribute included four photos of Charles and William -- two throwback pictures, and two semi-current ones.
Over on Queen Elizabeth II's page for The Royal Family, the well wishes read, "Happy Birthday to The Duke of Cambridge! As the second in line to the throne turns 40, learn more about His Royal Highness."
The post also shared fun facts about William, including his official birth time, university accolades, and family life. "His Royal Highness now undertakes a number of charitable activities and projects, with a particular focus on the environment, young people and mental health," the thread of info on William concluded.
"Wishing The Duke of Cambridge a very Happy Birthday!" a message on The Queen's Green Canopy page read. "HRH attended the illumination of the Tree of #Trees at Buckingham Palace during the #PlatinumJubilee weekend celebrations The Duke met schoolchildren who have taken part in the #queensgreencanopy RFS Junior Forester Award."
Ahead of his milestone birthday, William shared some information on a current philanthropic endeavor he's working on with The Big Issue. According to the Twitter bio for The Big Issue, it is "a social enterprise and weekly magazine sold by homeless or vulnerably housed people."
William shared, "I have always believed in using my platform to help bring attention and action to those who are struggling and I commit to doing what I can to shine a spotlight on this solvable issue not just today, but in the months and years to come."
Recalling an impactful moment he had with his mother, the late Princess Diana, as a kid, William added, "I was eleven when I first visited @PassageCharity with my mother and since then, homelessness has stuck with me as an issue I want to fight for."
On Sunday, William shared a photo with his three children -- Prince George, 8, Princess Charlotte, 7, and Prince Louis, 4, -- in honor of Father's Day in the U.S.
"Wishing a Happy Father’s Day to fathers and grandfathers across the world today!" he wrote.
As for how he'll be celebrating his 40th, royal expert Katie Nicholl tells ET that it will be a "private affair."
"The Cambridges will not have anything overly flashy or showy," she says, adding that any celebration will be "behind closed doors" with friends and close family.