Existing user databases had this field for ages and nobody bats an eye

This is a perfectly reasonable position to take - but you have to be clear that when you're saying that no software with an administrator controlled date of birth field (systemd-userdb, FreeIPA, OpenLDAP, 389-ds and more) can qualify as "Free" because of the risk of people using it to implement bad laws, you are inherently saying that people must use proprietary software to fill that niche.

It is the problem of the jurisdiction, if it restricts people. It is not the problem of Free Software. And we cannot fix this by restricting Free Software, obviously.

That depends. We can still endeavour to give people the best possible experience within the restrictions that do exist.

For example, here in the EU, regulations limit the 5-GHz-band WLAN channels open to the public. While the WLAN modems in our computers can theoretically use all the WLAN channels available anywhere, the Linux kernel supports a mechanism that lets us stick to the ones we're actually legally allowed to access here. In your logic, the authors of the WLAN support in the kernel should simply refuse to implement such a mechanism because it caters to a “problem of the jurisdiction”, and WLAN users in the EU should either accept jeopardy if we're caught using the taboo channels or just not use 5-GHz WLAN with Linux at all until we can get the laws changed. Obviously, both those alternatives are inferior to the one we actually have, namely accepting the restrictions (which frankly aren't that onerous) and enjoying the use of 5-GHz WLAN without having to worry about radio measurement vans outside our houses.

For the time being, nobody knows what an “age-attestation” infrastructure should look like (and the requirements are likely to vary by jurisdiction , anyway). In the end, however, I would much rather have a free Linux system that gives me the best possible experience in the face of any requirement for “age attestation” in my jurisdiction, than no Linux system at all because some guy somewhere on the Internet has decreed that ideological purity makes Free Software incompatible with such a requirement and that therefore developers should desist from addressing the issue at all on pain of receiving death threats and other harassment. Given this, a way of specifying one's date of birth in a standardised and well-understood place like the systemd user database is obviously not the worst idea if the alternative is that every other program must come up with their own version of /etc/birthdays.

It would clearly be even better if the laws were such that Linux would not have to provide an age-attestation infrastructure in the first place, but given that various places in the world may well end up with that requirement, it makes sense to consider how best to help Linux users in such an environment. In addition, even in the continuing absence of age-attestation requirements, having the possibility to store one's date of birth in a standardised and well-understood place on the system might come in useful at times – as the authors of all sorts of non-systemd user database schemes have apparently already figured out for themselves long before the age-attestation issue even came up, so there should clearly be no problem with systemd merely catching up with the rest of the pack.