Her government and subjects are having gun salutes, fireworks and tributes in Parliament in her honour, and televised retrospectives are being broadcast of a royal life that has stretched from the Roaring '20s to the Internet age.

The queen was born Princess Elizabeth on April 21, 1926, and became queen at 25 upon the death of her father, King George VI, in 1952. A majority of Britons have lived under no other monarch.

British Prime Minister David Cameron said the queen "has lived through some extraordinary times," from World War II to the moon landing, the end of the Cold War and the advent of peace in Northern Ireland.

Cameron led tributes Thursday in the House of Commons to the monarch and her "unshakable sense of duty," pointing out that the queen had provided counsel to 12 British prime ministers and met a quarter of all the U.S. presidents in history.

"Her Majesty has been steadfast — a rock of strength for our nation, for our Commonwealth and on many occasions for the whole world," he declared.

At dusk, the Parliament building will be lit up in the red, white and blue of the Union Jack.

The queen is spending the day at Windsor Castle and will greet well-wishers on a walk through the town west of London. Hundreds have lined up hours beforehand, carrying cakes, cards, balloons and Union Jack flags.

LONDON: On Thursday, Queen Elizabeth II became the first British monarch to reach the age of 90. It's just one of many milestones the queen has marked in her nine decades. Here's a look at some other numbers regarding Britain's queen:

• Elizabeth has reigned for 64 years and 75 days. On September 9, 2015, she became Britain's longest-reigning monarch, passing her great-great-grandmother Queen Victoria.
• She is the world's oldest monarch since the death of Saudi Arabia's King Abdullah last year.
• She has had 12 British prime ministers serve during her reign, from Winston Churchill to David Cameron.
• She has met 12 US presidents, from Herbert Hoover (after he had left office) to Barack Obama —more than a quarter of all the US presidents since Independence. The only president during her reign that she did not meet was Lyndon B. Johnson.
• She has traveled more than 1 million miles (1.6 million kilometres) on official trips, visiting 106 of the 193 current official members of the United Nations.
• She has visited Canada 22 times —the largest number of trips to any nation.
• She has four children, eight grandchildren and five great-grandchildren.
• She has cut back on her official duties in the last few years, but Elizabeth still conducted 177 official engagements in the last year.