Not sure if this is a COPPA issue at all if the situation involves a minor accessing a 18 and older site. I do not believe the website is required to affirmatively check the age of everyone entering it.
But given the lack of details here, it is difficult to say.
This is not legal advice but a general comment on society based on a limited set of hypothetical circumstances. No one should act or refrain from acting based on these comments without seeking appropriately licensed legal or professional advice. The author disclaims all liability in respect to actions taken or not taken based on his comments.