The certificates are generated once for each document. So every 10 years for a national ID.
The information is then signed using the certificate with the helpg of your local device. Since the government is the CAA, the receiver can quickly check if a valid certificate was used, thus whether the information is valid.
The certificate is saved on the chip which is embedded in the ID, preventing the user from altering the data.
> You can't know how old someone is without knowing who they are.
With this scheme you can. The system guaranteed that the ID used is either a certain age or above/under 18.
> Otherwise they can just use someone else's ID.
Just like in the real world. That is why with increasing stakes, say a bank account, additional information or tests might be needed.