Ok you obviously can't do research, I'll save you the trouble.
This, BTW IS Chicago Minicipal Ordinance rather thanh state law. Utility knifes BTW can have blades longer than 2 inches, which would otherwise be prohibited to under 18s.
c) No person under the age of 18 shall carry a utility knife on his person, or in the passenger compartment of a motor vehicle. This prohibition shall not apply to a minor using a utility knife for a lawful purpose (1) in his residence, under the immediate supervision of his parent or legal guardian; or (2) in a classroom, at the direction and under the immediate supervision of his teacher; or (3) in his place of lawful employment, at the direction and under the immediate supervision of his adult employer or an adult supervisor. For purpose of clause (3) of this subsection (c), "place of employment" includes an employer's motor vehicle used to transport the employer's tools and equipment, as well as a site where the employer is performing any lawful work
Notice how, one the one hand, there is a prohibition against the carrying of a utility knife, closely followed by exceptions for specific lawful purposes. Walk away from this argument with what little dignity you still have left my friend.