For the United Kingdom specifically, I've suffered the misfortune of reading the Online Safety Act, and this kind of age estimation is both mentioned and permitted by the Act. (Not a lawyer blah blah blah)

Part 3, Chapter 2, Section 12(4) specifies that user-to-user service providers are required to use either age verification or age estimation (or both!) to prevent children from accessing content that is harmful to children. Section 12(6) goes on to state that "the age verification or age estimation must be of such a kind, and used in such a way, that it is highly effective at correctly determining whether or not a particular user is a child."

Part 12, Section 230(4) rules out self-declaration of age as being a form of age verification/estimation.

So I suppose it'll come down to whether or not Ofcom deems Discord's age estimation as "highly effective".

[Part 3, Chapter 2, Section 12(4)]: https://www.legislation.gov.uk/ukpga/2023/50/part/3/chapter/...

This is unrelated, but something I find interesting is that Category 1 user-to-user services (of which Discord is one, as per The Online Safety Act 2023 (Category 1, Category 2A and Category 2B Threshold Conditions) Regulations 2025) are required by Part 4, Chapter 1, Section 64(1) to "offer all adult users of the service the option to verify their identity (if identity verification is not required for access to the service).".