AFAIK there has to be assurance given that someone is of age. This can be done directly or by a third party. Gambling sites tend do this by requiring ID to be uploaded which has to be verified before use. Said ID will have to carry something on age/date of birth...
I have never gambled - online or off - so I don't know how it works in detail. But the level of assurance (of child protection) gained under the Online Safety Act is ludicrously low. So low as to make the Act dangerous by giving false assurance.
I think assurance levels for gambling are higher than that for porn. Its also in the interest of gambling operators to be clean. They are licensed and could lose it if they breach said licence.
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).