As others have said, it depends on which state "the deed" occured in. Also, the fact that it could have been different states gives near automatic reasonable doubt, unless there is some form of proof. (in other words, they could claim there were in one state where it was legal, such as Mississippi)
Personally, I've often found some of these laws to be absurd. I'll gladly condemn any adult who goes after minors, BUT, when we're talking two kids in their late teens and very close in age, that's just nuts and no place for the law.
In Arizona, it's even crazier. For example, if two seventeen year olds are having sex, that's basically legal. But let's assume one is five days older than the other. If they are in a long-term relationship, the one who turns 18 first can get in real hot water, along with the added prize of lifetime membership in the registered sex offenders club.
Worse, there is an exception for both still being in high school, but it's a proactive defense, NOT an automatic out. In other words, they can still be charged, and its then up to the jury to consider the defense.
This has occurred to teens just days apart in age when in a long-term relationship. One of the nastier bits of this law is that an ongoing sexual relationship prior to the 18th birthday of the eldest is considered presumptive evidence of said relationship occurring once the eldest turns 18.
There have been prosecutions over this. It's rare when they are within a year's age, but it has happened at paren'ts urging, usually whn its the girl who is younger and the parents don't like the guy. I shudder to think was could happen to a gay couple under these circumstances, as the jury might not be so inclined to consider the proactive defenses.
Jack, my sincere coldolences on the loss of your mother.
BTW, you said you see a story in this somewhere. Well, there is one story that I know of here on GA that had exactly this Arizona law as a major plot point. Been as it's relevant, and I did research the law for it, I'll link it. It's called "For the Love".