Did I get this right? Is the MC in this story having sex with his stepsister and three of her friends on page five, and THEN turns eighteen on page six?
I left a comment under the story asking this same question, but... the comment is gone now.
Since you don't cite the story, how would anyone else know?
Sounds like the author knows the line might be a little bit shaky, if they're removing your comment, but as StillStunned says, you can always report it.
It's possible that a few stories escape the screening net of the moderators every now and then. No system is flawless.
Use the 'Report' button to notify the moderator and be specific about the details (this helps the mod to find the problem faster). They will take action as soon as possible.