No, it’s not illegal for a 15 year old to have a gun. It is illegal for the 15 year old to do what he did with iy.
1931
750.234f Possession of firearm by person less than 18 years of age; exceptions; violation as misdemeanor; penalty.
Sec. 234f.
(1) Except as provided in subsection (2), an individual less than 18 years of age shall not possess a firearm in public except under the direct supervision of an individual 18 years of age or older.