The actor Rupert Everett has expressed concerns about parents who seek medical treatment for children who question their gender.

The 57-year-old, who is gay, revealed he wanted to be a girl as he grew up and dressed exclusively as one when he was a child. He suggested that Caitlyn Jenner, the TV personality and Olympian who was formerly Bruce Jenner, had made a mistake in undergoing gender transition and had “no clue” what being transsexual involved.

Everett, currently starring in the BBC1 show The Musketeers, told the Sunday Times magazine: “I really wanted to be a girl. Thank God the world of now wasn’t then, because I’d be on hormones and I’d be a woman. After I was 15 I never wanted to be a woman again.”

He believes parents who “get medical” are scary, saying: “It’s nice to be allowed to express yourself, but the hormone thing, very young, is a big step. I think a lot of children have an ambivalence when they’re very young to what sex they are or what they feel about everyone. And there should be a way of embracing it.”

He also spoke of how his former girlfriend Beatrice Dalle, the French actor, thought she might have been pregnant with his child. Everett told the magazine: “She and I often talk about what would have happened if we’d both had this child. And I would have loved to have had that child, actually. God, it probably would have been in rehab by now.”

He also revealed that he was “turning into his mother”, and planned to move back in with her in Norfolk.

• This article was amended on 20 June 2016. The Press Association misquoted Everett as telling the Sunday Times he opposed gender reassignment operations for children. In fact, Everett expressed reservations about hormone therapy. In the UK, gender re-assignment surgery is only available over-18s.