Young Adults save the world in so much of the fiction aimed at Young Adults. And when this science fiction comes to the big screen, YA fiction becomes PYA, full of PYTs.

It’s a future where the boys have peach fuzz and the girls perfect hair, perfect teeth and perfect makeup — even when they’re Maze Running, Hunger Gaming, or simply Divergent.

Thus, Chloe Grace Moretz, Alex Roe and Nick Robinson are the Pretty Young Adults struggling to stop “The Fifth Wave.” If only they’d succeeded.

It’s not quite the most laughable of the many eye-rollingly derivative entries in this overused genre. “The Giver” still takes that prize.

But from its love triangle centerpiece, to the adults and institutions that cannot be trusted, the child soldiers trained for Mortal Combat — military style — and the post-Apocalyptic future where only the prettiest survive, “Fifth Wave” never escapes the genre joke that it is.

Moretz (“Kick-Ass”) is Cassie, our narrator who misses “the Cassie I was.” The one before she became “the Cassie who kills.”

We meet her After the End, or Beginning of the End. Aliens have invaded, and all she has is a teddy bear, a backpack (apparently full of skin and hair care products), an assault rifle, and skinny jeans.

She’s trying to get back to her kid brother, who is training with Liev Schreiber, Maria Bello (in her scariest hiarstyle ever) and the Army to hunt The Others. Those are the aliens.

Along the way, she must choose between the boy she crushed on in high school (Nick Robinson) and this new stubbly, sometimes shirtless hunk (Alex Roe) she meets on the run. My favorite was the Emo/Goth Girl super soldier (Maika Monroe). But really, they’re all just as ridic.

The best elements in “Fifth Wave” are the flashbacks — showing the aliens arriving in their “District 9” spiky spaceships, then subjecting the human race to various special effect cataclysms (“waves”) as they drive us toward extinction.

Moretz rarely suggests the terror this should bring, but she does manage the grief that losing her family would cause. It’s just that every time she’s around a boy, those overripe lips tremble and the screenplay pauses for an Awkward Teen Moment — courtship, you see.

It’s all so obvious and (unintentionally) laugh-out-loud funny.

Seriously, if you’re not five steps ahead of “The Fifth Wave,” you need to have yourself tested.

MPAA Rating: PG-13 for violence and destruction, some sci-fi thematic elements, language and brief teen partying.

Cast: Chloe Grace Moretz, Liev Schreiber, Maria Bello, Nick Robinson, Alex Roe, Ron Livingston, Maika Monroe
Credits: Directed by J Blakeson , script by Susannah Grant, Akiva Goldsman and Jeff Pinkner,  based on a Rick Yancey novel. A Sony/Columbia release.

Running time: 1:52