I think requiring age verification to access porn is a good idea
In isolation, nobody is going to argue otherwise.
The problem is, quite simply, that there is no jurisdiction that has either the authority or the means to actually enact a meaningful age restriction on accessing porn, given that the content is stored on servers in every corner of the planet.
So this legislation comes to pass in the UK, fine and dandy, you've restricted access to an utterly insignificant portion of all of the porn freely available on the internet, while the vast bulk is still instantly accessible due to not being subject to these new restrictions.
Lobby all the governments of the world to act in unison? Sure, go ahead, but you are wasting your time. Aside from the intransigent governments who simply do not care, you also have those where government restrictions on freedom of expression are contrary to their constitutions, so they wouldn't even entertain the idea in the first place. While you still have territories where suppliers of pornography are not subject to these laws, that is where they will store their content, and that is why your kids will still be able to access it completely unfettered, even though your government has enacted your age restriction laws.
What can be done in the meantime, is better, more effective, and more competent monitoring and policing of usage by those responsible for the minors, including asking if it is prudent to actually give a minor a smartphone with unfettered access to the web in the first place.
Government is essentially rendered impotent by the very nature of the internet itself, and the fact that all the governments of the world do not, and never will sing from the same hymn sheet in unison. What isn't rendered impotent is the individual, who absolutely can take prudent steps to limit what children have access to, if they so choose, and if they simply make the effort.