There’s a cute coarseness that lets “My Old Ass” live up — or down — to its title and its standing as “An Aubrey Plaza Comedy.” That’s Plaza’s brand, after all, and she is the title character.

But this sentimental story of a child of 18 awakened to the realities, responsibilities and future regrets of that moment in life by her older self isn’t an Aubrey Plaza comedy. It’s a Canadian “last summer before college” romance set in cranberry country made quaint by its setting and the adorably outdated term we’d use for the nature of that romance — “bi-curious.”

Maisy Stella is our heroine, Elliott, a brash, self-absorbed teen who “can’t wait to get out of” the lakeside/cranberry bog-side Ontario village where she grew up. When we meet her, she’s blowing off a planned family birthday party to A) recklessly tool around the lake on her outboard motor skiff, B) make out with her store-clerk girlfriend Chelsea (Alexandra Rivera) and C) go camping to sample some “shrooms” with her besties Ro and Ruthie (Kerrice Brooks and Maddie Ziegler).

“My life is finally about to start,” she crows. What she doesn’t see is what’s about to end.

That’s what this dose of mushroom tea does for her. It earns her a “Hey, freak” visit from “39 year-old you,” dryly played by the queen of too cool for school, Plaza.

After a few “middle aged” backhanded compliments and protests about how they look nothing alike — Gapped teeth? “WEAR your retainer!” — the life lessons turn more serious.

Little brothers? Connect with them before you go. Mom? “BE NICE to her.”

Remember, “The only thing you can’t get back is time.” Oh, and one last thing.

“Can you avoid anyone named ‘Chad?'”

It isn’t until Elliott’s sobered-up and stumbles into Dad’s new summer hire for the cranberry harvest that she takes in what has really happened. The willowy cranberry charmer is an undergrad named Chad (Percy Hines White). And that new contact on her cell phone, the one labeled “My Old Ass?” That’s her future self, reachable by cell for further life advice and clues about what her future holds.

But who answers their cell any more? Plaza’s character recedes into the background as Elliott finds herself confused, with feelings and urges to work out, rewarded for finally reconnecting with her loving family, and punished for all that she’s already missed, the “changes” that were already underway, the “last time you will ever” do things that matter, that you want to ensure linger in the memory.

Actress (“What If,” “The Secret Life of the American Teenager”) turned director (“The Fallout”) Megan Park’s script is entirely too on-the-nose in many little ways. Naming your gay heroine Elliott after Canada’s most famous transgender performer Elliot Page, the designer girlfriend and best friends, having one of Elliott’s little brothers obsessed with actress Saoirse Ronan, making the beau who turns her head a super sensitive long hair are all easy, early draft touches to give a script its flavor.

Let’s show girls urinating a few times for extra “edge.”

That flavor could have come from the unusual locale, the local farming money crop and Elliott’s family, all of them given short shrift here.

When “My Old Ass” works, it’s comically judgmental, droll in the ennui the older feel about the young and sentimental about what you’re missing out on by focusing on the impulse of the moment.

It could have been a modern “Peggy Sue Got Married,” but it only comes close to that youth-revisited classic in one or two moments.

There’s no arguing that “Nashville” alumna Maisy Stella gets a “The next Florence Pugh” stardom making showcase here. But sweet as it is, “My Old Ass” could have used a bit more “old ass” to make the sentiments stick and the sense of losses to come more palpable.

Rating: R, drug abuse, sexual situations, profanity

Cast: Maisy Stella, Aubrey Plaza, Percy Hines White, Alain Goulem and Maria Dizzia

Credits: Scripted and directed by Megan Park. An MGM release.

Running time: 1:29