Most 15yr olds know that, too. But teenagers, including 18yr olds, are prone to
not making sound decisions. He is a legal adult, but he is not an adult in the same sense as a 25yr old, a 35yr old, or a 45yr old. and so I don't think 18yr olds should be held to the same standard. That doesn't mean he should be treated like a "kid", because we should expect better of an 18yr old, but maybe not the same level of expectations of adults that have real life experience.