Flood of tributes for Queen Elizabeth on her 90th birthday

WINDSOR, England -- Fate unexpectedly made her queen. Duty and endurance have made her an institution and an icon.

Queen Elizabeth II turned 90 on Thursday as Britain's oldest and longest-reigning monarch, drawing crowds of well-wishers and floods of tributes to the stamina and service of a woman who can claim to have given her name to the age.

Britain is living, Prime Minister David Cameron said, in the "modern Elizabethan Era."

The queen usually spends her birthday privately, with most of the pomp and ceremony reserved for an official birthday that's marked in June.

But Thursday's milestone brought an outpouring of public goodwill.

Thousands of fans greeted the queen on a tightly choreographed walkabout near her Windsor Castle home, while elsewhere, her government and subjects held gun salutes, fireworks and speeches in Parliament, and televised retrospectives offered scenes from a royal life that has stretched from the Roaring '20s to the internet age.

"Her Majesty has been steadfast -- a rock of strength for our nation, for our Commonwealth and on many occasions for the whole world," Cameron said as he led tributes in the House of Commons.

He praised the monarch's "unshakable sense of duty," pointing out she had provided counsel to 12 British prime ministers and met a quarter of all the U.S. presidents since Independence.

Her record is all the more remarkable because she was not born to be queen.

When Princess Elizabeth was born April 21, 1926, her father was a younger son of the king and not expected to reign. His older brother took the throne in 1936 as Edward VIII -- but abdicated the same year to marry his divorced American lover, Wallis Simpson.

Elizabeth's father became King George VI, and at 10, she became heir to the throne.

When she was 21 -- almost five years before she became queen -- she promised the people of Britain and the Commonwealth, "My whole life, whether it be long or short, shall be devoted to your service."

She kept the promise, and it has struck a chord with people in Britain and around the world.

"She's such an icon and a real role model for the children of today. And I think everybody should respect her for all the years that she's given for her country," said Donna Werner, an American tourist from New Fairfield, Connecticut, who came to greet the queen in Windsor, just west of London.

Thousands lined the streets of the town carrying cakes, cards, balloons and Union Jack flags.

The band of the Coldstream Guards played "Happy Birthday," and royal fans snapped cellphone photos as the queen, clad in pale green, greeted local dignitaries, townspeople and tourists.

By her side was 94-year-old Prince Philip, her husband of 69 years, with whom she has four children, eight grandchildren and five great-grandchildren.