Question: does your next of kin have to be over 18

Answer: No, In some legal systems, rights regarding inheritance (which imply a decision-making capacity -- for example, in a medical emergency -- where no clear will or instructions have been given, and where the person has no spouse) flow to the closest relative (regardless of the age, with a representative appointed if a minor), usually a child, a parent or a sibling. However, there are people without any close adult relatives and, in such a case, decision-making power often flows to a niece or nephew, first cousin, aunt or uncle, or grandparent.