Sometimes people confuse adult pornography that uses adults dressing or posing as minors (16 yrs old-looking actors, etc.) for legal teen themed sexually-explicit content. In one case it's unethical marketing, and in the other images of a crime seen. Big difference.
This is further complicated by ads and SPAM advertising “barely legal teens.” While most child advocates wish that these were illegal and that no adult content providers could use teen-themed sexual content or market “barely legal” content, in the US this is (generally) not child pornography.
It might be repulsive and violate your networks’ terms of service, but is not illegal.
For everyone: Report what you find to the network where you found it. They will then report it to the cybertipline in the US or other tipline around the world.
For industry risk managers: This is why setting clear rules and guidelines is important. And given the safe harbor requirements, it may be better off if you report items that aren’t technically child pornography just in case. Your policy professionals will advise you better on this.
And these images may be just as disturbing to you as actual child pornography is. So, make sure you follow up with them to understand your role better and what they can do to help you maintain a safer and healthier work environment.