A minor may be defined as someone under the age of 21 or 17 depending on the cruise line and itinerary.
To ensure the safety of any minor travelling with them, most cruise lines ask any adult taking responsibility for a minor to have the correct legal documentation to show they are permitted to take the minor on the cruise. They may also ask for the document to show you can sign waivers and other such documents for the child.
In the case where by an adult is taking a minor and they are not the Parent or Legal Guardian, they will request documentation to show the adult has the permission from a Parent/Legal Guardian to do so.
Each cruise line may have different rules around these situations. We advise you to check with the cruise line's website at the earliest opportunity so you have time to get the necessary documents.
An example of what a cruise line may ask you to provide (Royal Caribbean, Celebrity and Azamara Cruises policy) can be viewed here.