Setting aside the ridiculous nature of this move towards OS-level verification, NixOS (and Guix) is the last distro to worry about when it comes to age verification.

Why? Given the nature of how NixOS works (config-driven), the maintainers have plausible deniability: if push comes to shove, they can shift the burden to users and have them enable the age verification service as part of their NixOS config.