I don't see a problem with someone 16 dating their SO after their SO turns 18.
I DO have a problem with someone 21 or older dating someone under 18. Even with knowing that with varying levels of intelligence, there are people who might be mentally able to cope better than others. That isn't the point.
We don't give people under 18 the rights of an adult. They are considered their parents responsibility in all matters. So you find them sexually appealing? And your state gives you a pass, depending on age, to ignore the fact that before and after you're done fucking them they ARE NOT AN ADULT? Its a bad message to send to underage people that if they want to be treated as an adult before they reach the age of legal adulthood they need to have sex with older people. That their sex partner gives them validity. Its a disgusting echo of our past that an older person's sexual interest trumps all.
Now as for the 18 year old dating both a 16 year old and a 23 year old? You're all right she may not understand poly well. Neither did I at the very adult age I began exploring it. And I didn't understand everything concerning monogamy when I was 16 either. I made mistakes and life went on. I'm sure this 16 year old will make her mistakes as well no matter what style of relationship her BF has with other people. The 23 year old with alters and legal issues? Just play it safe and stay out of her life. It can be trouble you don't need if you don't.