London ? Prince William turned 21 on Saturday, proving himself to be the model of a modern royal: He wears jeans, loves fast motorbikes, does his own grocery shopping and holds tight to his independence.

True, William’s coming of age brought new responsibilities for the second in line to the throne. The day was marked in traditional style with an official poem and newly minted gold coins.

But the prince also made sure it was celebrated his way, ordering the royal family — including his grandmother, Queen Elizabeth II — to ditch the usual formal state function in favor of an African-themed costume party at Windsor Castle.

William, who is teaching himself Swahili and has been on safari in Africa several times, said he wanted to avoid a “sterile” official function and have his relatives let their hair down.

“I thought it would be quite fun to see the family out of black tie and get everyone to dress up,” he said.

“My grandmother may be slightly apprehensive as to what she’s going to wear, and what’s going to happen, but she’s very much looking forward to it. She’s very positive toward the whole thing.”

Earl Spencer, the brother of the late Princess Diana, was one of the early arrivals for the “Out of Africa” party, driving through the castle gates dressed in a light-colored safari-type suit.

Comedian Rowan Atkinson, better known as the character “Mr. Bean,” and his wife followed, both dressed in linen suits. One partygoer was dressed head-to-toe in a furry lion suit topped with a gold crown.

A palace spokesman declined to reveal what members of the royal family would wear.

Britain's Prince William plays a set of drums as the Shakarimba band, flown in from Botswana to play at the prince's 21st birthday, rehearse in the Queen's Guard Chamber at Windsor Castle. William chose an Out

“They have been wanting to keep it a secret even from each other so it will be a surprise,” a spokesman said on condition of anonymity.

The party for more than 300 family and friends was strictly off-limits to the media.

In an interview to mark the occasion, the sporty prince described his desire to support charities, following in the footsteps of his mother, Princess Diana. He also defended his father, Prince Charles, from critics and described the values he lives by.

“My guiding principles are to be honest, genuine, thoughtful and caring,” William told the Press Association, the British news agency.

He also quashed newspaper speculation he did not want to be king. In the weeks before his birthday, newspapers speculated William would shun his royal duties and intended to move to New York after graduation.

“It’s not a question of wanting to be, it’s something I was born into and it’s my duty,” he said. “But those stories about me not wanting to be king are all wrong. It’s a very important role and it’s one that I don’t take lightly.”

William said he was greatly influenced by his visits to homeless hostels as a teenager with Diana, who was killed in a Paris car crash in 1997.

“I learned a lot from it, more so now than I did at the time,” William said.