Some dread the big 4-0 - but not Prince Harry, we're told. As he heads into this new decade, he's looking ahead with excitement.
While that may not be surprising after the turmoil of the past few years, you have to remember that parts of his 30s were pretty good.
In May 2015, when Harry was still 30, I interviewed him in New Zealand as he prepared to leave the army. It was one of those pinch-yourself interviews; I'd not spent that much time with him at this point, but he really opened up - telling me of his dreams to find someone to settle down with and how he'd "love to have kids right now".
These were dreams that in the next four years would come true. We had the excitement of his wedding to Meghan at Windsor, people talked of the "new Fab Four" as the couple stepped out with William and Kate, and there was a new royal baby to coo over as Archie arrived.
His Invictus Games had become a huge success, alongside his Sentebale Aids charity. It looked like Harry, by his mid-30s, had found the perfect love match to make the Royal Family feel truly relevant on the international stage.
What happened next filled millions of column inches around the world as the fairytale collapsed and he and Meghan moved to America. No one could have predicted how far Harry would go in his book and television programmes, and the breakdown of his relationship with the Royal Family continues to run.
It has been an incredibly painful few years. Some inside the palace had hoped King Charles's cancer diagnosis would start to mend things, but Harry's anger and fear about his security arrangements when he comes to the UK apparently continue to be a sticking point.
When I asked on Friday whether Kensington Palace had anything to say about his birthday, I was told "no".
Buckingham Palace didn't have anything to say either but seemed to leave the door open for some kind of public acknowledgement to appear.
Harry's next big public appearance comes in just over a week's time, when he is due to join the team from The Diana Award and young people to discuss mental health issues facing their generation in New York.
Their chief executive Tessy Ojo, who has worked with Harry and William for the past 24 years, told me this is a milestone for Harry that represents "a moment of opportunity and forward momentum", as he enters "a phase of life where his experiences, values, and purpose align more strongly than ever".
The next decade is pivotal, she said, "because it offers a chance for him to further solidify his role as a global changemaker, not just in terms of philanthropy but in how he shapes new approaches to social impact".
This new chapter "will likely reflect an even more profound dedication to creating a world focused on the next generation, a world where children and young people, irrespective of where they are born, can thrive in a society grounded in equity, mental wellbeing, and opportunity for all", she added.
People often talk about the disappearance of the old Harry. The lad, the man, who liked a laugh.
You still see glimpses of it, especially with his banter at Invictus military events, and through his work for Scotty's Little Soldiers and WellChild, where you can't deny how brilliant he is with children.
When I was told he is planning a weekend break with his closest mates for his birthday, I'll admit I immediately thought of that infamous trip to Las Vegas in his 20s.
But most people realise eventually they have to grow up - and Harry has had to do a lot of that more recently without his father or brother, whoever you want to blame.
And of course, without his mother Princess Diana, who died when he was so young.
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There is one conversation I had with him before he met Meghan that has continued to stick in my mind. We talked about whether he'd ever move out of London or even leave the UK.
It was clear he'd thought about it a lot, but at the time his support for his grandmother, Queen Elizabeth, was keeping him firmly close by.
We now know he would go on to take that leap of faith, and this will be a big decade. One where we'll really see what life outside the royal fold looks like long-term for Harry and his young family - and if reconciliation will come.