The problem is the need to set an arbitrary age limit. It is a necessary evil. Children and adolescents should be protected from predators. The age 18 is sensible because it is also the age of majority... the age when children officially become adults and are thus legally capable of taking responsibility for all actions.
The only problem is that age is very arbitrary. A seventeen-and-a-half-year-old is not much different from an eighteen year old.
But still it is necessary to have some law in defense of kids. Since not all people develop the same it is hard to do. But certainly you would not want a twelve year-old in a sexual relationship with an adult (or even another twelve year-old). Eighteen might be a bit odd, but it is logically consistent in that it is the age that the law recognizes adulthood (17 would work just as well since it can be age of emancipation).

As for how much the difference in age is when both people are adults... That's no one's business but your own. The gray area is when there is high school senior dating a junior... I think there are caveats in the laws to protect people in such situations, but it varies by state.

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The Tall Man, Phantasm III