1 of 23
2 of 23
For his first birthday, the Royal Family gushes over William's six new teeth and his first teetering steps. "He falls over a lot," Diana's father Earl Spencer says of his grandson.
3 of 23
Is 4-year-old William a royal pain? Now a big brother to Harry, nearly 2, Charles and Di's firstborn gains a reputation as William the Terrible. "William's very enthusiastic about things," Diana says, before adding, "But the bad luck about being No. 1 is trial and error, so we're open-minded about William."
4 of 23
Nearly 7, the King Tot outgrows his "William the Terrible" phase and matures into a "natural leader" to classmates and his little brother, whom he advises on proper royal etiquette. "Stop it, Harry," he commands when he catches his brother sticking his tongue at the press. "That's very naughty."
5 of 23
When William, nearly 9, is rushed to the hospital after being accidentally hit on the forehead at school, Diana sticks by her son's side during an emergency surgery, as her husband leaves his family for the opera. The press lashes out at Charles for being an "absentee parent."
6 of 23
7 of 23
"I now treat him as an adult," a palace aide tells PEOPLE of William, who's nearly out of tween-dom. While his brother has the luxury of being a second son, William – who is destined to be King – "is very, very aware of his future."
8 of 23
At 14, William is heralded not only as the heir apparent to the throne, but a budding British heartthrob, having made his first appearance in the pages of a teenybopper magazine. "He's quite a good-looking lad," says the editor of Smash Hits, "and has potential as a pinup."
9 of 23
At 15, William bids farewell to his mother, who dies in a fatal car accident in Paris at 36. With his brother Harry, 12, at his side, he marches behind Diana's coffin, which he topped with a white wreath of tulips. "We all care desperately for you today," Diana's brother Earl Spencer says to the boys during his speech. "We are all chewed up with sadness at the loss of a woman who wasn't even our mother. How great your suffering is, we cannot even imagine."
10 of 23
With their mother gone, Prince Charles – who was so frequently criticized for his cold parenting style – focuses his attention on William and Harry. "Charles's No. 1 priority right now is the boys," a Palace insider says of the Prince's commitment to comforting his mourning sons.
11 of 23
William's 16th birthday turns out to be a bittersweet occasion, as it's the growing royal's first birthday without his mother. Holding up Diana's legacy, the teen prince "has replaced his mother as the royal star," royals author Judy Wade told PEOPLE.
12 of 23
On the anniversary of their mother's death, William and Harry "are doing surprisingly well," according to the Majesty's Seward, in part because of Charles's hands-on parenting. "The prince has warmed to the task of being a single parent," says royals correspondent Peter Archer. "Charles and the boys are a close family."
13 of 23
William and Harry distinguish themselves from the stereotypes of royalty, excelling at Eton College and maturing under the protective eye of their father. The Queen says the boys together "will take the monarchy into the 21st century," according to a royal insider.
14 of 23
Now 18, William gives the press a glimpse into his final year at Eton, where he excels in geography, biology and art history, as well as water polo, soccer and polo. "I've really enjoyed being able to go about Eton as just another student," he says. Adds his uncle Earl Spencer, "He has some of his mother's magic with people."
15 of 23
As Diana's boys grow into men, William and Harry forge a tighter bond over royal duties, polo and country weekends with dad. "They need each other in terms of moral support," says a friend of William's. "Since the death of their mother, even more so."
16 of 23
Now a freshman at Scotland's St. Andrews University, Diana's shy boy wants to skip the fanfare of his royal welcome and ultimately fit in. "It will get easier as time goes on," William says. "Everyone will get bored of me."
17 of 23
The differences between William and Harry become more apparent as the press ramps up its coverage of the brothers. As William studiously handles the responsibility of being England's future king, Harry relishes in his role as second son by partying, fighting and reportedly cheating on a graduation exam. "William and I try to be normal," Harry says. "It's very difficult, but we are who we are."
18 of 23
19 of 23
Having spent nearly five years together, will William make Middleton his future bride and queen? "She is princess material and 21st-century queen material, I have no doubt about that," says veteran royals writer Robert Lacey.
20 of 23
Nearly 10 years after Princess Diana's death, William, 24, and Harry, 22, plan a star-studded concert and memorial service to honor their late mother. "There's not a day goes by I don't think about it," William says of Diana's passing, while Harry adds, "To us, just two loving children, she was quite simply the best mother in the world."
21 of 23
Engagement speculation turns feverish as William and Middleton reach the seven-year mark – but will they cap their long-running romance with a royal wedding? While there's no confirmation, someone close to William says he'll propose, but "it's just a question of when."
22 of 23
After making a rare public appearance together at a friend's nuptials, William and Kate ignite a new round of wedding forecasts that come true within the month as the couple of eight years, both 28, announce their royal engagement on Nov. 16. "Everyone is absolutely delighted. He has never had eyes for any other woman since Kate came on the scene," says an old friend. "She is going to make a wonderful wife, a wonderful princess."
23 of 23
How exactly does an ordinary English girl dazzle a prince and put herself in line to become the future queen of England? For Middleton, it's her mix of grace, poise and classic style. "I'm willing to learn quickly and work hard," she says of taking on her new role, with her fiancé pulling double duty as groom-to-be and royals coach. "William's a great teacher," she added, "so hopefully he'll be able to help me along the way."
The Harvey Weinstein Scandal: All of the Hollywood Figures Who Have Spoken Out Against Movie Mogul
Kelsea Ballerini & Morgan Evans Took Shots at the Altar, Plus More Stars' Wedding Secrets
Star Tracks: Celebs on Social
Slime! Sheet Masks! and Even More Fads and Viral Moments That Dominated 2017
Celeb Quotes of the Day: Tom Hanks, John Stamos & More