One technical problem with doing this on the header level is that SPAs will fetch content gradually with multiple requests, so the browser _and_ said applications would need to account for filtered content being fetched halfway through the page load.
A more cynical take is that policymakers either actively want or are OK with mandating digital ID-based authentication in service of age controls, so any kind of privacy-preserving age controls will be either treated as horribly onerous or a safety risk when someone figures out how they can be circumvented.
One technical problem with doing this on the header level is that SPAs will fetch content gradually with multiple requests, so the browser _and_ said applications would need to account for filtered content being fetched halfway through the page load.
A more cynical take is that policymakers either actively want or are OK with mandating digital ID-based authentication in service of age controls, so any kind of privacy-preserving age controls will be either treated as horribly onerous or a safety risk when someone figures out how they can be circumvented.