The occasion is hardly going unnoticed in London, where remembrances in the form of cakes, candles, cards and other tributes are being left outside the gates of Kensington Palace, home to Diana before she was killed in a 1997 car crash at age 36.
"She would've been so popular still. Everyone would have been here to help celebrate," Kathy Martin, 49, a childcare worker from Australia, tells the Associated Press. "We'll never get to see her grow old."
Martin said part of the lure to Diana's residence on this day was the princess's age.
"Marilyn Monroe was an icon, Grace Kelly was an icon, and Princess Diana was an icon," said the visitor. "They all died young. They'll be remembered as princesses – beautiful, radiant, princesses."
Like Marilyn, Grace and JackieSharing a similar sentiment is Diana's biographer, Andrew Morton, The author, delivering his own remembrance in Friday's Daily Express, writes: "When I I ponder where Diana would be now if she was turning 50 today, two other women come to mind; Princess Grace of Monaco and Jackie Onassis. All three eventually left their homelands or social circles to become the independent, international icons of popular imagination."
He calls all three women "voluntary exiles, finding themselves in spite of their backgrounds and careers and in the process shattering the existing iconography surrounding them."
In Morton's eyes, Kelly, whose son Albert is marrying this weekend, left an Oscar-winning movie career to wed Prince Rainier of Monaco. Dignified former first lady Jacqueline Kennedy married "rough-hewn Greek shipping tycoon" Aristotle Onassis. And Diana rebelled against the Palace by leaving and then divorcing Prince Charles, which Morton calls the crown's "biggest crisis since the 1936 abdication".
Had she lived, Morton speculates, Diana today would be at home in New York or the Hamptons, watching her son and daughter-in-law on TV during their overseas visit.
Also, he imagines, "she would be looking carefully to see if Catherine, whom she would have taken under her wing, was following her advice on how to deal with the big public events. Not that Catherine, who has taken to the spotlight as if to the manner born, has needed much quiet coaching."