Adult = 18 or over. Adult is the opposite of minor ... under 18.
Minors are allowed to possess long guns and pistols, too, as long as the minor complies with HRS 134-5. Basically, if they are target shooting or hunting with a hunting license, a person 16 or older can possess a long gun. If possessing a pistol while hunting, the pistol needs to conform to rules for hunting with such firearms.
I think DLNR or the hunter's ed curriculum has a list of pistols and calibers that are legal to hunt with.
Minors under 16 in possession of a firearm while hunting or at the range have to be accompanies by an adult.