"You could volunteer at any age. Underage boys could enlist with the permission of their parents, but there are numerous examples of boys who ran away from home and enlisted in a Confederate organization. If the surgeon examined the boy and found him physically capable of field duty, he was in service. Parents had to go through the court system or appeal to a senior military authority to have him released."

A person could legally enlist *with* the consent of the parents, at a younger age...but it was illegal to enlist under a certain age without that permission. That is why a court case could be filed, and the underage boy released from duty. This is at least partially based in contract law, whereby a minor cannot legally contract. When the military authority released the boy from duty, this was called "age disability", just as being too old was an "age disability".

As I understand it, before the Jr. Reserves was created, it was illegal to join (without parental consent) if the person was under age 18. This is the important change enabled by Section 6. A 17 year old could now volunteer, apparently without parental consent. However, I don't see that it allows anyone younger than 17 to enlist without parental consent.