If it says "Under 18" in the signature, then it means that the NemID has been issued to a person under 18.
However, this does not necessarily mean that the person still is under 18 years old.
NemID certificates are valid for 3 years. If you get your NemID issued when you are 17, you will get a certificate called "Under 18" - and 3 years later, your certificate will remain the same, even though you are over 18.
You can make sure that the person is not under 18 by checking their CPR number.
We recommend anyone who is over 18 to update their certificate by going to NemID's webpage. It should only take a few minutes, and then you will have a regular NemID.