Queen Elizabeth II marks her 91st birthday

— -- Queen Elizabeth II celebrates her 91st birthday today. She is expected to spend the milestone at Windsor Castle.

Clarence House released a photo this morning of the Queen and Prince Charles in 1952 to mark the occasion.

Princess Elizabeth Alexandra Mary was born April 21, 1926, in Mayfair. She became presumptive heir to the throne 10 years later with the abdication of her uncle, King Edward VII. At 21, Queen Elizabeth married Prince Philip in a ceremony at Westminster Abbey.

On her 21st birthday, Queen Elizabeth made a speech that would define her reign.

"I declare before you all that my whole life whether it be long or short shall be devoted to your service and the service if our great imperial family to which we all belong," she said.

She welcomed her first child, Prince Charles, in 1948.

At 25, on February 6, 1952, Queen Elizabeth became the monarch when her father, King George VI, died at Sandringham. Queen Elizabeth, who was in Kenya at the time, returned home to the overwhelming life that would mark her reign.

Her coronation was held a year later, in 1953.

The queen became the nation's longest reigning monarch in September 2015 when she surpassed her great-great grandmother, Queen Victoria. Earlier this year, on February 6, Queen Elizabeth achieved another milestone with her sapphire Jubilee, serving 65 years on the throne.

Her birthday will be marked with a 41-gun salute in Hyde Park, a 21-gun salute at Windsor and a 62-gun salute at the Tower of London. This year's birthday is less extravagant than the historic celebrations that swept the country last year for the Queen's 90th birthday, when she also became the longest reigning monarch.

Last year's yearlong celebration culminated with a horse pageant at Windsor Castle, a street picnic stretching from Buckingham Palace down London's Mall and Queen Elizabeth lighting a chain of more than a 1,000 beacons to mark her birthday throughout the U.K.

Queen Elizabeth usually spends her actual birthday quietly and in private.

A second official birthday celebration, Trooping The Colour, is held in June when the country can participate in the monarch's birthday. The tradition dates back to the 18th century when King George II allowed his birthday to be celebrated in June instead of November because of the poor weather in the winter months.

Her Majesty, who has generally been in good health, gave the country a scare when she became ill last Christmas. Queen Elizabeth came down with what the palace described as a "heavy cold" and flu, causing her to miss the annual Christmas service at Sandringham.

Queen Elizabeth's nearly three week absence from public life sent the nation into a period of anxiety over their beloved monarch. She recovered and was recently seen riding her horse outside Windsor Castle and feeding elephants with Prince Philip at an engagement.

Her busy schedule, which includes weekly audiences with Prime Minister Theresa May, sees her take on upwards of 400 engagements a year, even in her 90th year. She is sovereign of 15 Commonwealth realms, including Canada and Australia, and is head of the Commonwealth.

In the past year, though, Queen Elizabeth has slowly scaled back her pace a bit, allowing the younger generation to take on additional royal duties as a subtle changing of the guard is taking place.

Queen Elizabeth handed over 25 patronages to other members of the royal family, including the Wimbledon tennis championship to Princess Kate.

While Prince Charles has assumed additional responsibilities as heir to the throne, more attention has focused on Prince William, Princess Kate and Prince Harry, who are increasingly standing in on behalf of their grandmother.

The arrival of William and Kate's two children, Prince George and Princess Charlotte, and the popularity of Queen Elizabeth has cemented the future of the British monarchy after a period of uncertainty in the 1990s. In her 65 years on the throne, the queen has modernized the monarchy and left it stronger than ever.