Around 6,000 sites allowing porn in the UK will start checking if users are over 18 from Friday, according to the media regulator Ofcom.
Dame Melanie Dawes, its chief executive, told the BBC "we are starting to see not just words but action from the technology industry" to improve child safety online.
She told BBC Radio Four's Today programme that "no other country had pulled off" such measures, nor gained commitments from so many platforms, including Elon Musk's X, around age verification.
But the BBC understands some major porn sites operating in the UK do not seem to be requiring age checks as of Friday morning.
And experts remain sceptical about the effectiveness of Ofcom's age checks and how easy it may be for people, particularly children, to circumvent them.
