IIRC, it used to be puberty. However, we now have good record-keeping, typically know a person’s age, and thelaw prefers numbers, and parents freak out if their child’s made to drop trousers by investigators. And modern puberty’s shifting earlier than it used to be.
Anyways, 18+ is good for “about everybody’s past puberty” and mature enough to decide if they want to bang a senior citizen (hey, maybe they’re after the inheritance). And good enough to nab the 40-year-old predators wandering into the elementary school for a date. Or nabbing a parent abusing their kid.
However, when it comes to the teenagers themselves, that’s not so good. After all, “normal” for teenagers is going after other teenagers. (Warning: I am NOT a lawyer.) IIRC, most states have some age-based window here. So 14 & 15 would not be in trouble with the law. Another issue with teenagers and these laws… with the prevalence of cell-phone-cameras, a teen can easily get into trouble for taking pictures of themselves! (That’s a topic for a different thread.)
Back to original topic, I’ll apply the [minor1] tag for non-sexual nudity, because I’d rather be safe than sorry in disclosure. (And if the character’s naked, I’m sprinkling in words to remind… ie, comments about the variously exposed bits from time to time.)