From your very link, it looks like teen sexual assaults are exactly what they DO NOT want reported. It would destroy the system. Here:
Any person who knows, or has reasonable cause to suspect, that a child is abused, abandoned, or neglected by a parent, legal custodian, caregiver, or other person responsible for the child’s welfare, as defined in this chapter, or that a child is in need of supervision and care and has no parent, legal custodian, or responsible adult relative immediately known and available to provide supervision and care shall report such knowledge or suspicion to the department in the manner prescribed in subsection (2).
The bolded part is NOT a teenager.
And yes, we would be legally obligated to report child abuse even by a stranger ADULT. Read:
Any person who knows, or who has reasonable cause to suspect, that a child is abused by an adult other than a parent, legal custodian, caregiver, or other person responsible for the child’s welfare, as defined in this chapter, shall report such knowledge or suspicion to the department in the manner prescribed in subsection (2).