All domestic and foreign flights with unaccompanied minors under the age of 18 should require an assigned flight attendant with all airlines for the safety of children traveling to and from parents/family. My children were minors and flew with health conditions and should there be a medical emergency, the flights their father got them did not require unaccompanied minor assignments for kids over 14 and should there be a medical emergency, there would be no adult to make health decisions for the minor if they need to reroute the flight for emergency hospital visit.
It should not be various ages for every airline. Children cannot make their own medical decisions in an emergency and an adult should be there for them. Therefore, a policy for all minors under the age of 18 should be assigned as an unaccompanied minor with a flight attendant in charge of their well-being.