1 I buy a device for my child and I set it up and enter his birthday, say he is under 13
2 I buy me a device , setup my account and enter my credit card to use the Store , the OS now is 100% sure I am an adult
3 The browser on our devices knows our ages now
4 13+ , 18+ webpages will mark this in the header , the browser on the child phone knows to not just allow the child to click "I am 18+ old" , the browser on my device would check my settings and say if I am a religious guy I tell the browser not to show the pages to me.
5 if you can't or won't tell your OS or browser your age either send your ID card to all websites or don't go there, big websites will respect local laws so maybe you can have luck with a VPN or some small website that does not care about local laws in a different country.
I suggest this idea as an alternatives to having to send ID card copy to each websites, I am not advocating for UK or Australian laws to be made default everywhere.