This can be used to have zero-proof knowledge of "over 18" or "not over 18". So they don't really get your age, except that you are in two broad ranges.
I think anon's point is that it could be used for other attributes in the future, like your nationality or... your social credit score (don't worry, it only proves that your score is over or under 500).
If you get enough signals like that you can often narrow down a very large cohort of people to an individual.
First it's 'over 18?', then it's 'over 25?', and then 'biological sex?', 'employed?', 'enjoys posting on HN?', 'active in the early morning?' and after half a dozen questions, all with binary answers that are safe individually, you can zero in on a 23 year old woman who has a job and posts on HN in the morning.
Ask a few dozen questions like that and you'd be able to sieve an individual from a group of millions, especially if they're unlucky enough not to be absolutely typical.
The point perhaps is that these things enable discrimination based on extremely gross grained and defective criteria - in some ways the least relevant parts of your identity.
First it's 'over 18?', then it's 'over 25?', and then 'biological sex?', 'employed?', 'enjoys posting on HN?', 'active in the early morning?' and after half a dozen questions, all with binary answers that are safe individually, you can zero in on a 23 year old woman who has a job and posts on HN in the morning.
Ask a few dozen questions like that and you'd be able to sieve an individual from a group of millions, especially if they're unlucky enough not to be absolutely typical.
Obviously if you see a bunch of proofs for known circuits coming from the same IP address then yeah, you can infer a bunch of info from that metadata.
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