One way to do this off the top mf my head would be
HN issues a unique number (say 4096 bit) to you when you create an account
You send that number to your identity provider along with confirming proof of age
The identity provider signs that the number is valid and posts it to a public source
HN downloads a list of 4096 bit numbers posted in the last 5 minutes and confirms the one associated with your account is on the list
HN will know that "Identities-r-us.com" has proven your age, but nothing else
IRU know you had to age approve a site, but there are many sites downloading the lists so they don't know which one