Although you could assume that the person is younger based on their posting style, it is a little bit of generalization to assume someone's age based on the intelligence of their post content. Maturity to some degree can be derived from post content, and if a user repeats their behavior, I would assume myself that lying about their age would be to their detriment. If someone who claims to be 19, for example, is posting spam repeatedly, they should know better and will not be given the benefit of the doubt as often. As for enforcing COPPA if a user reveals there age to be below it, I would think that is up to staff. If the user is being belligerent and uncontrollable, then kicking them out would be agreeable. If they were not, then since they provided two ages to the forum, it is indeterminable legally in terms of which age provided is the correct one, so letting them stay would be fine.