Keira Knightley went to prom with a girl.

Asked who her prom date was, she said: "It was a she. Neither of us got invited by anybody else. No guys came anywhere near us. But she had black tie because it was black tie."

And the 'Laggies' star is still friends with her unnamed date.

She added: "She actually texted me at lunch. She is over from New York visiting her mum in England and wondered if I was there to have dinner with her."

During the interview, Keira also admitted she still doesn't feel like an adult - until she has to pay her bills.

In a game of 20 questions with her 'Laggies' co-star Sam Rockwell for E! Online, she said: "I mean, when the tax bills comes in that sorts of does it really and trying to figure that stuff out. Paying bills, that does it."

While Keira may not feel like an adult, she has reached many of the criteria which a new Generators Z report from Lucozade Energy says make people feel like they have finally become a grown-up.

According to the study, things such as moving out of parents' home, getting married, making financial decisions and having a job after university are all signs of adulthood, as well as things like enjoying cooking and gardening, favouring pubs over clubs, watching the news and enjoying sight-seeing on holiday.

To read more, including how 24 is the average age in which young people feel grown up, and to see the accompanying filmed content featuring a selection of young people across the UK discussing the issue, please visit www.generatorsz.tumblr.com.