Niall was the adult in this situation, it was his obligation to find out the age of the person he was having sexual conversations with. If Niall was lied to about the age, at least he did the simple job of making sure he wasn't ERP-ing with a minor as far as he knows. He didn't ask for age and evidence suggests that Niall knew Ivory was underage since Ivory posted his twitter link in the Discord DM, the link had a preview of Ivory's bio which said he's 16. Even then, Niall saw this as nothing more than fantasy roleplay, this only makes things worse as he still thinks that he did nothing wrong here.