"His life will change hugely when he becomes Prince of Wales," says one source close to the Duke. "He will be in charge of the Duchy of Cornwall, a billion-pound business empire, and will be expected to carry out far more royal engagements, especially abroad. It's part of the reason he has never really had a grand plan that might have involved signing up for something that might take 20 years to achieve, because he has always known this is coming down the line."
William has been as much a victim of events as of circumstance: he did settle on a plan to spend the second half of his 30s tackling the world's ills using the star power of the Cambridge/Sussex Fab Four, only for Harry and Meghan to walk away. Friends say Megxit left him having to "start from scratch", with Covid further hampering his attempts to define his public role.
Yet he is not a man who is likely to spend his 40th birthday fretting about the state of his legacy. Those close to him say he is "more comfortable in his skin than ever", and does not regard his birthday next Tuesday as a significant moment, even though he has joked that turning 40 is a "daunting" prospect.
William has long made it clear that family comes first for him, a generational shift from his father and grandmother, and in that respect he can be rightly proud of his record as a husband and a father. He has three thriving young children who stole the show during the Queen's Platinum Jubilee celebrations and a solid, happy marriage to a woman who has grown to be a perfect royal companion.
The Duke of Cambridge's limbo years might well turn out to be his best.
"He and Kate are in the happiest period of their lives now, before the weight of the world lands on their shoulders," says a friend. "They try to not think about what's ahead."
The couple have made one major decision to coincide with their 40th birthdays (the Duchess's was in January), which is to shift their home base from Kensington Palace to Windsor during the summer so that their children can all attend the same co-educational school when 4-year-old Prince Louis begins his formal education.
Their move to the four-bedroomed Adelaide Cottage on the Windsor estate is all part of their plan to give their children as normal a childhood as possible (as well as being closer to the Queen) though their longer-term plan is likely to involve a move to Windsor Castle, which is not the Prince of Wales's intended future home. By migrating to Windsor now, the Cambridges will ensure stability for their brood.
When he takes over his father's current role, William is expected to carry on using Kensington Palace as his working London base, with Anmer Hall in Norfolk continuing to provide a country retreat for the Cambridges.
Yet, unlike his father, who regards Birkhall on the Balmoral estate as his true home, closely followed by Highgrove in Gloucestershire, William has no real affinity with Scotland, and spends little time there. The future of Balmoral as a royal residence is open to question; as the Queen's personal possession, it would be in Prince Charles's gift to donate it to the nation, keeping Birkhall as a more modest bolthole north of the border. Her Majesty's other personally-owned grand house, Sandringham, is likely to remain in the family and will almost certainly become the Cambridges' Norfolk home in the long term.
It is easy to forget that William's determination to be a hands-on father with a real-life job meant that he remained a part-time royal worker until he was 35. Having served as an RAF Rescue pilot, he flew air ambulances in East Anglia until as recently as July 2017. Since then, he has determined that his work should focus on a limited number of what aides call "big bets", rather than the relentless round of tree plantings and plaque unveilings that have spread royal work thinly across a bewildering number of good causes.
Homelessness, mental health and the environment are the Duke's biggest concerns, and the Earthshot Prize – giving million-pound grants to five projects each year for a decade – is the closest he has come to his own version of a Prince's Trust or a Duke of Edinburgh's Award.
William's achievements at 40 will inevitably be compared with his father's at the same age (as well as the Prince's Trust, Charles had set up Business in the Community, the Prince's Foundation and others), but allies of the Duke of Cambridge rightly point out that Charles had the added status, wealth and infrastructure that comes with the job, making it far easier for him to make an impact.
Where Charles was intellectually restless, seeking stimulation from mentors like Laurens van der Post and Armand Hammer, shaping Highgrove and Clarence House in his own image, William is content for his life to revolve around home and hearth.
Charles was also in the depths of a failed marriage when he was 40, having restarted his affair with Camilla Parker Bowles after his relationship with Diana, Princess of Wales foundered. In that respect, William has done rather better than his father.
Happily, the two men are perhaps closer now than they have ever been, partly because adversity has drawn them together (aside from Megxit, the two acted as one in insisting that the disgraced Duke of York be banned from the Garter ceremony this week) and partly because of William's increasing need to shadow his father as he comes ever closer to taking over the Duchy.
"In the aftermath of Megxit, William has started to appreciate his father more and Charles trusts his son's judgment more than ever," says one insider close to the Duke. "You saw during the Platinum Jubilee a flavour of how the Prince of Wales is more involved with his grandchildren than ever before, and William is less reliant on the Middletons for that sense of family."
Having been compared to his mother for his entire life, William is finally becoming his father's son – and as he prepares to become the new Prince of Wales, he might finally understand the meaning of that badge he gave to his father all those years ago.
