Prince William at 40: Settled as a Dad, Thriving as a Royal — but in 'Pain' Over Prince Harry Rift

As Prince William spends his landmark 40th birthday working at Kensington Palace ahead of a private family celebration, PEOPLE's chief foreign correspondent Simon Perry, who has reported on the Duke of Cambridge for 25 years, speaks with those who have worked with him and reflects on the man and prince he is today.

Hitting a milestone age is a perfect time to look forward — and, sometimes wistfully, back. "I'd like to be 21 again," Prince William joked in an interview with The Big Issue magazine this week. "The big four-zero. Getting on a bit now."

Yes, and with plenty to be proud of.

His 11-year marriage to Kate Middleton, which was built on the foundation of their friendship at the University of St. Andrew's, has blossomed into one of the great modern royal partnerships. It has also seen the births of their three beloved children — Prince George, 8, Princess Charlotte, 7, and 4-year-old Prince Louis — who were pictured joyfully embraced alongside him in a portrait released for Father's Day.

He has found contentment in securing the balance he was always looking for: Having a private family life while recognizing that he has to share some of it with the public – moments of which were clear during the recent Platinum Jubilee celebrations. When the couple headed to Cardiff, Wales, on the morning of the Jubilee concert, it was clear to those of us there how they were gently teaching their eldest two children what it's like to work on the royal frontline. As he lovingly placed his hand on George's shoulders and brought Charlotte in close to talk to a young woman in the crowd, William helped them take some of the first steps that mirrored his own introduction to royal duty at a similar age.

Away from the cameras and mindful of the difficulties that confronted his parents, Princess Diana and Prince Charles, decades ago, he has been determined that his own family life be as normal as possible — and with his wife Kate, rightly praised for being a solid impeccable spouse as they face the next decades together, he has the best partner.

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But it's with deep sadness among those who know the prince, that one partnership that has been lost is with his brother Prince Harry. Their estrangement is an ongoing sore that shows no sign of easing soon and is a source of "pain and hurt" for both William and Charles, a close insider says.

As he steps up more and more in his role as future King, William credits his mom —who died 25 years ago in August — with guiding much of his work. "Ever since I came here [to homeless charity The Passage] with my mother, homelessness has stuck with me as an issue I want to fight for. I want to do a lot more," he told The Big Issue. This week's interview was a signal of a key area that will remain central to the work-life of William the heir, the future Prince of Wales, and, ultimately, monarch.

Alongside climate change, the environment (and his ambitious Earthshot Prize), and mental health, William is carving out ways to make a difference via his role as key convenor and advocate. "He has not shied away from big societal issues," says Tessy Ojo, who has worked with the prince at the Diana Award and through his cyberbullying task force. "There has been a boldness and he will sometimes go where people might not expect them to go – like when we set up the cyberbullying campaign and he said to the tech companies, 'You can't put profits before people.'"

He has also managed to navigate that difficult terrain of appealing to politicians and decision-makers while not overstepping the mark and becoming party-political on the world stage too — whether it's urging President Xi Jinping in China to join the ban on the illegal trade in endangered wildlife or deftly treading a diplomatic path in visits to Israel and the neighboring Disputed Territories and, in 2019, Pakistan. "In his early 20s, I always had to pinch myself to remind me that he was still a very young man. He already had an extraordinary maturity," says Charlie Mayhew, CEO of the Tusk Trust charity. "That, I suppose, comes with a knowledge of his destiny. He has developed into a really valuable statesman for this country and for conservation and the environment."

It was no accident that William's speech at the Platinum Jubilee concert earlier this month was on behalf of the environment — a cause he inherited from his father. "He was saying to the next generation, 'We hear you' and we will use our platform to do what we can,'" a close insider says.

Of course, it hasn't always been plain sailing publicly. The couple's tour through the Caribbean in March had an undercurrent of controversy as visits were canceled due to protests about colonialism. The Jamaican Prime Minister told William to his face that it was his intention to push for a republic. Then, there were the accusations that the royals were "tone deaf" as they stood on the back of LandRover to leave a military parade. (For their part William and his team have stressed frequently how they were listening and learning from what they were told during the tour.)

He's sure to have discussed some of the fallout from the tour with his father, Charles, 73. The rift with Harry (the brothers spent no meaningful time together during the Platinum Jubilee weekend) has arguably helped bring William and his father closer. After some difficult years when they didn't see always see "eye to eye," as a close source admits, their relationship is said to be much improved. Naturally thrown together as plans develop for the future, with William one day taking over much of what his father does today, they have also been united over the rift with the younger prince and over Prince Andrew's non-role in the royal family.

Soon, William, Kate and their children will move to Windsor, putting them at the heart of the family heritage — both personally to his frail grandmother Queen Elizabeth, and metaphorically so, as the 1,000-year-old castle shares the family's name.

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The future challenge for William, alongside Kate, will be to make the monarchy relevant for the millennial generation amid some of the new pressures that will uniquely fall on them as the sole representatives of the younger working royals in the family. They will have to forge a way forward while helping the monarchy adjust and understand "the social and cultural challenges of the age," a close insider suggests.

Tessy Ojo doesn't doubt the prince has the talent to do so. "He is a thoughtful person, and I also feel that in some sense he is more aware of modern-day Britain and is able to go where the pain is, where the people are. It is important that the monarchy stays in touch with the people and the best way of doing that is to constantly listen, which is a skill I know he has and has demonstrated," she says.