I think it is worth mentioning that age and sex is encoded in the social security number.

There are exceptions where the encoded birth date will be wrong (like immigrants with unknown birth dates) or dates where there are more people than the 4 digits that encode checksum validation and gender can handle.

I learned a few days ago from a friend, when the other leak at the university was reported in the Danish media, that the "random" part is only 4 digits and as a bonus is sequential. So if you register in the country as foreign citizens together with your partner those are likely sequential.

Then again, in my country some municipalities handed out numbers starting with your birth year....

A good time to remember that cramming data into your identifiers will come back to bite you... It hasn't really been necessary since databases replaced library catalogs anyway.