Princess Charlotte claims "she’s 16" whenever someone asks her age, according to her father, the Duke of Cambridge.
Prince William admits his daughter – who turned six on Sunday – is very confident and jokes she is already a teenager.
He said: "If you ask her, she says she's 16 ... Charlotte says, 'I'm six now, I'll do what I want' …They grow up very fast."
William also discussed Charlotte's recent birthday celebration, adding that she'd had a far more enjoyable birthday this year as an easing of lockdown measures meant they could at least have one other family join them.
Asked if she had a lovely birthday, William replied: "She did thank you very much, yes.
“Unfortunately for her obviously last year there was no birthday party because of lockdown but this year we had one other family, so that was it.
“So it wasn't a party as such but we made it as fun for her as possible."
Charlotte is the Duke and Duchess of Cambridge’s only daughter.
They also have Princes George, seven, and Louis, three.
William was speaking at Babcock International Vehicle Engineering in Walsall, who were praised for managing to keep their production going during lockdown by using Covid-19 safe systems.
They were also involved in the design process for NHS ventilators.
While in the Midlands, William also met some of the players from the football team he supports, Aston Villa.
He visited their Warwickshire training to officially open their new £6 million high performance centre.
During the visit he showed his support for his team, who play in claret and blue, by wearing a blue suit and facemask with a claret jumper.
While there he spoke to some of his favourite players, including the Aston Villa captain Jack Grealish.