LONDON — On April 21, 1947, Princess Elizabeth marked her 21st birthday with a live radio speech in which she paid tribute to her subjects across the Commonwealth and the British Empire.
"I declare before you all that my whole life, whether it be long or short, shall be devoted to your service," the U.K.'s future queen said from Cape Town, South Africa. "I shall not have the strength to carry out this resolution unless you join in it with me, as I now invite you to do."
These words still ring true for her supporters on Thursday, her 90th birthday.
For millions of people, the longest-serving monarch in British history is a part of the fabric of U.K. society and a steadfast constant in their lives.
"For many of us, in our lifetime, the queen is all we have known," Katie Nichol, royal correspondent for the Mail on Sunday newspaper, told NBC News. "She’s on the stamps that we lick and put on our envelopes, she’s on the coins that we pay for everything with … she is an institution, she is a national treasure."
The queen is marking her birthday by meeting well-wishers outside Windsor Castle, west of London. She is also lighting the first of more than 1,000 beacons throughout the country in honor of the event.
"As a nation we want to thank our queen for her boundless duty," said Ingrid Seward, editor-in-chief of London-based Majesty magazine. "A lot of people don’t get near that [age], but she is still out there doing that, looking lovely in her brightly colored clothes, shaking hands, and talking to people."
Journalist and royal commentator Roya Nikkhah added: "She came to the throne in her early 20s and I think in this country the public recognize and admire and appreciate the fact that she has given pretty much her entire life to service."
The Windsor event is no anomaly; despite her age the Queen carried out 306 engagements in the U.K. and 35 abroad in 2015, leaving the younger royals in her wake. Prince Harry, Prince William and Duchess Kate managed only 198 domestic and 94 foreign engagements between them last year.
"I think it’s remarkable," 84-year-old royal enthusiast Tim O’Donovan said. "How many ladies of 90 work as hard as she does?"
O’Donovan, a retired banker from Windsor, has marked every royal event for the past 37 years by adding clippings of the occasion to his scrapbook.
The queen was voted the most popular U.K. monarch of all time last year and most British people are in favor of the monarchy.
But not everyone agrees.
Almost a quarter of Britons said they did not think the country would still have a hereditary monarchy in 100 years' time and 9 percent said they were against the royals altogether, according to a YouGov poll last year.
Representing that viewpoint is the campaign group Republic, which wants to "see the monarchy abolished and the queen replaced with an elected, democratic head of state." It calls the royal family "a broken institution."
Dr. Anna Whitelock, a researcher at Royal Holloway, University of London, says that while many people respect the monarchy, some question its place in a modern society.
"It belongs in many ways to a bygone age," she told NBC News. "There are some who would questions its relevance — what it symbolizes is privilege, elitism."
Furthermore, there are signs that the queen may need to start to take things a bit slower following her birthday.
"I think that Buckingham Palace are very keen to say it’s business as usual," Nikkhah said. "But there is no doubt that, at 90, both the queen and her aides see it as a game-changer, and there is only so much we can expect of the monarch at age 90."
British media reports suggest that some of the charities and causes championed by the queen will be handed over to younger royals.
Duchess Kate will take on the queen’s patronage of the Wimbledon tennis tournament, Prince Harry will look after the governing body of rugby in England, and Prince Charles is expected to travel on more of the foreign tours in her stead.
The next generation may be starting to take on more royal duties, but they acknowledge that when it comes to the royal family, the queen continues to reign.
On tour in India this past week, Prince William spoke about his grandmother.
"I am incredibly lucky to have my grandmother in my life," he said. "As she turns 90, she is a remarkably energetic and dedicated guiding force for the family."
William added: "It is very important that everyone here tonight understands that I am here as the queen’s representative. She may be my grandmother, but she is also very much the boss."