Porn sites can use third parties or check documents directly. Another means of verification is charging a credit card. Only people over the age of 18 are allowed to have a credit card. In this case the site is relying on the card provider to warrant age.
None of which offer verifiable assurance. I am sorry, I have been professionally reviewing public sector assurance for several decades. Third party assurance cannot be relied on. On what basis does government assess assurance of third parties? Third parties who want to allow access for profit?
Gambling sites need to verify more than just age. Gamblers can be placed on industry wide exclusion lists so they need to verify against these as well. (I spent two years working on facial recognition product for gambling Learnt a lot about.it).