By Caroline Frost
BBC News Profiles Unit
William is enjoying life away from London
Prince William's 21st birthday marks the coming of age of a thoroughly modern royal. He carries the hopes of the British monarchy, but shows no sign of rushing to embrace his destiny.
In his personal attributes, he appears a typical, if privileged, young man. He skis, swims, drinks too much at parties, enjoys the attention of young ladies and, as the recent episode of road rage testifies, occasionally drives too fast if he thinks he can get away with it.
But of all the youths in the land who take afternoon tea with granny, not many stride through the gates of Windsor Castle to do it. Walkabouts, protection officers, balcony appearances and postage stamps all attest to the status of a future king.
Prince William of Wales can put off using his HRH for as long as he likes, but he remains indisputably royal.
Inherent regality: William's 21st birthday is marked by a stamp
Although it was Earl Spencer who expressed the wish for his nephews' souls to "sing openly", it was always the combined efforts of William's parents that ensured he had a fun, contemporary childhood.
Diana broke with royal tradition when she took her newborn son with her to Australia. She made William and Harry queue up for a hamburger in Kensington High Street. And she took them on a water-log ride, when they famously got soaked with other children.
The princess also took them away from their royal circle to meet less cosseted members of society. She wished them to realise that "everyone is not rich, has four holidays a year, speaks standard English or drives a Range Rover".
The Prince of Wales has certainly provided his son with his share of royal trappings. William shares his father's love for blood sports, and the pair often defy popular opinion to go hunting and shooting on the Queen's estates.
Diana wanted a normal childhood for her sons
In an interview to mark his 21st birthday, William called himself "a country boy at heart".
Since William's early days at school, Prince Charles, perhaps mindful of his own miserable days growing up in the glare of the media, has helped to protect his son's freedom.
Horror of the cameras
When William entered Eton, the Prince of Wales struck a deal with the press whereby, in return for limited access, his sons were otherwise left alone.
This has seemingly worked. William has been studying at one of Scotland's fashionable universities, St Andrews, for two years, but so far only the occasional picture of the young prince with shopping bags has emerged.
Prince Charles struck a deal to help protect his son's privacy
His horror of the cameras dates from his teenage years, when he would shout at photographers to "leave my mummy alone".
High tabloid value
He was, by all accounts, horrified by the way the media charted his parents' marital discord, and against the royal couple's use of the papers as their own point-scoring chessboard.
And then, in September 1997, he grieved in the full glare of the world's attention when, aged 15, he walked behind his mother's cortege.
Since the tragedy that befell the Princess of Wales, William has made it his business to conduct his life away from the all-inquisitive lens, and his birthday interview revealed only that he's inherited a family trick of using lots of words to tell us nothing very much.
In 1997, the world watched William grieve for his mother
This means, though, that he has little practice at dealing with the ruthless press. No doubt contrary to his intentions, his reticence has also raised his tabloid value and engendered a public thirst for the smallest detail about him.
In addition, with the looks of his beloved mother and the action-man lifestyle of his father, William's own Garbo-esque enigma has only enhanced his appeal.
In a poll of 18-to-24 year olds taken last year, Prince William proved the most popular choice to be the next king. This, before he's made a speech, signed any documents or aligned himself with any causes.
Can William at 21 carry this burden of monarchy, yet remain an easy-going, confident young man? His mother thought so, saying "My boy's got a good brain, considering how hopeless both his parents were".
But the truth is that, while he continues to study, swim, drink and develop, we don't actually know. And, as William tucks into his birthday cake on Saturday, it seems he's happy to keep us guessing.