North Dakota is "shall issue". You do need the local LEO to "sign off", but that is not used to deny people unless there is an articulable reason. I have given the ND test to many many folks, both from ND and from other States; I have never had a denial.
I believe that most (all??) "shall issue" States have the out that the local Sheriff can deny the application if they believe the applicant is a danger to themselves or others. That does not make them "may issue", unless that is the way you wish to define it. That would change many "shall issue" to "may issue."
As to the age requirement, you always follow the law of the jurisdiction you are carrying.
I have had numerous <21 year old applicants from Minnesota that get the ND Permit...primarily for when they hunt. In these cases it also is true that the MN local LEO's might not sign off. Assuming they just refuse to sign off, it has no adverse affect on getting the ND Permit. It actually is fairly common for non-ND LEO to refuse to sign. The applicant then simply states that they offered the local LEO a chance to sign off, but they refuse. Bismarck will still process and issue the Permit.
Clearly, getting the ND permit to cover ND (unless you are under 21) is not your best choice. Get Utah, or New Hampshire...they are honored in ND and a better bargain and less hassle.