The star told a Cannes press conference she likes to make "bold" choices, as if she were at the start of her career.
Kidman said while she doesn't have to work, she does so because of her passion for cinema.
The star of The Killing of the Sacred Deer, in competition at Cannes, said her young children will be banned from seeing the dark psychological thriller.
Kidman has two grown-up children from her marriage to Tom Cruise and two younger daughters with her current husband singer Keith Urban.
"My (younger) children have very little understanding of what me or my husband do. Their lives are obviously far more important!"
She added: "I'm at that stage in my life where I still try to act as if I'm 21 and starting my career.
"At this stage of my life, I'm just trying to stay very bold and open and try things and support film-makers I believe in."
The Killing of the Sacred Deer features an eminent surgeon Steven Murphy - played by Colin Farrell -  who adopts a teenager into his family. Things start to go horribly wrong when the boy turns against Murphy.
Kidman plays ophthalmologist Anna Murphy, who is married to Steven.
It's directed by Yorgos Lanthimos, the Greek filmmaker behind indie hit The Lobster, which won the Jury Prize at Cannes two years ago. Farrell also starred in that film.
Kidman said she was "hypnotised" by Lanthimos' script.
She has four projects showing at the festival this year, but said: "That's a confluence of events - that's not something I was aware would be happening."
Her other Cannes showings are in The Beguiled and How to Talk to Girls at Parties, as well as the second series of TV drama Top of the Lake, from Jane Campion.
She singled out Campion for praise, saying she had been a friend of hers since the age of 14 and that "she discovered me, basically."
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The two English players shared the first four frames before O'Sullivan hit a 124 clearance to edge ahead in the semi-final in Cardiff.
O'Sullivan, a three-time champion, then made breaks of 94 and 88 to lead 5-2 before winning with a 78.
Australia's Robertson beat Northern Ireland's Mark Allen 6-4 to reach his second final, to be played on Sunday.
Robertson, the 2007 champion, had trailed 2-0 in the best-of-11 frame match, but won the crucial third frame after a foul by Allen, of Northern Ireland.
The world number three then won five consecutive frames to lead 5-2 and withstood a comeback by Allen to reach the final.
Speaking after the semi-final, O'Sullivan, 40, compared Robertson to cinematic cyborg law enforcer RoboCop.
"I have watched him through the UK Championships and he never showed emotion through the whole tournament, " O'Sullivan told BBC Sport, ahead of the best-of-17 final.
"It shows what an unbelievable professional he is. He is Robocop, he is like the new Steve Davis or Stephen Hendry."
Robertson, 34, believes O'Sullivan is the favourite for the title.
"I don't mind flying under the radar," he added. "It is nice, I can be the underdog."

The 24-year-old, whose current contract ends at the end of 2016, was offered a four-year extension by the Scottish Premiership leaders in January.
Forrest came through the youth ranks with the reigning champions, for whom he has made 144 appearances.
However, his outings have been restricted by a series of injuries.
Forrest, who has 13 Scotland caps, was missing from Celtic's squad for Saturday's 3-0 victory over Inverness Caledonian Thistle.
Celtic signed Manchester City winger Patrick Roberts on an 18-month loan deal from Manchester City in January and last week secured a summer move for 17-year-old midfielder Kristoffer Ajer from Norwegian club Start.
Deila admitted on Saturday that Celtic had too many midfielders on their books and that he would have to slim his squad at the end of this season.
Oscar winner Nicole Kidman says she still acts like she's 21 when it comes to acting roles.
[NEXT_CONCEPT]
Ronnie O'Sullivan beat Joe Perry 6-3 to reach the Welsh Open final where he will play Neil Robertson.
[NEXT_CONCEPT]
Norwich City and Queens Park Rangers are among clubs showing an interest in James Forrest after the Scotland winger rejected a new Celtic contract.