If there’s a small enough age gap, someone over 18 can date someone a little younger than them. For example, there’s a kid who goes to my school who is dating a girl in a grade above him. She is 18 and graduated, and he’s 17. It makes very little sense to prevent them from dating for a year just because she’s 18. Thus there are limited exceptions to the age of consent like these “Romeo and Juliette” laws you’re referring to