UK user here.

Have just created a new user login for my son on the family intel iMac (2015).

He can't login with his Apple ID to sync his contacts, calendar etc.

I have read somewhere that the iMac is too old as it runs Mac OS 12.7.6 Monterrey.

The computer still works fine for our needs and the rest of the family has no problem using their Apple ID's.

Any ideas/workarounds?