If you think the voltron paladins, who were legally in the garrison, are underage (Hunk, Lance and Keith),
then that means you think the military on Earth using child soldiers is a normal practice.
They are part of a military organization.
Don’t try to say “it could be a military school” cuz military school would be normal school with particular emphasis. They are training for real missions in space, and are called cadets. They are part of a military unit in-training.
I for one don’t think child soldiers would be a thing. And I’m creeped out that so many fans are so insistent that they’re children. I don’t want child soldiers in my Voltron.
SAY NO to the child soldier theory.
I have faith the creators never intended to portray child soldiers.