The Staten Island teen charged in the violent death of her boyfriend’s baby son shook the child so hard that he suffered brain damage, authorities said.

Cynthia Dubois, 18, allegedly shook 13-month-old Josiah Taylor earlier this week and then waited 20 minutes to call for help after he became unconscious. Dubois pleaded not guilty yesterday to charges of reckless assault and endangering the welfare of a child.