The phrase "overgrown child" refers to an adult who acts in a childish and immature manner, displaying behaviors that are expected of much younger individuals.
It used to be that kids were treated as mini adults, and now the pendulum has swung the other way and young adults are being treated (and acting) like overgrown children. (bonbonbreak.com)
I can't exactly blame him, he is cast entirely off type as a sort of overgrown child who may be mentally deficient. (alphaprimitive.com)