if we are being kinda serious, we would probably just go with either the systemd user metadata solution, or... (on a scale of insane to less insane):
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store it in a passwd comment (which means the finger command will now remotely leak age, but is backwards-compatible)
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have an SELinux (or similar) setup so that new users have more restricted permissions by default. While I would absolutely do this myself whenever I have kids, this is just really unnecessary for most everyday users. At the very least this obfuscates the reason why a user might have less permissions.
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make a generic, encrypted-by-owner file in /etc/, but this still has privacy issues if we need every account to state their age.
Obviously the best solution is just educating parents about managing parental controls, but we can't have nice things ig.
I wonder if using a hash-based system could work slightly better (on platforms where you can't edit image links) in these cases where a platform decides to stop serving to some users or go defunct. So that way, you can at least have a slight chance of 'fixing' an image after the fact.
It wouldn't work with most image share sites, but at least you could rehost it on a mirror yourself.