That summer before college can be strange, a moment suspended between life stages, and Elliott (Maisy Stella) is right in the thick of it. She’s turning 18 in the tiny picturesque community in Muskoka, Ontario, where her family has farmed cranberries for generations. She has a janky little motorboat, two best friends and a massive crush on the girl behind the counter at the local coffee shop. And she’s looking forward to heading to Toronto in a few short weeks to start the next chapter of her life.

Sounds familiar. You might even relate. But “My Old Ass,” written and directed by Megan Park, does not go in expected coming-of-age directions. It’s as much about reframing middle-aged regrets as it is a story about youth, love and possibility — and thus the emotional heft it wields is two-pronged.

Elliott belongs to a newish and very welcome variety of teen girl movie protagonist. For decades, these characters were mostly siloed into vapid types, the better for us, I guess, to “understand” them: Goths, cheerleaders, ditzes, bookish wallflowers, cool girls, bullies. Elliott, on the other hand, is funny, capable and comfortable in her own skin. She can drive a tractor and steer a boat, and also forgets to show up for her own birthday dinner with her family. She is very thoroughly 18, with as strong a sense of self as you can really have at that age, while also being kind of a jerk at times to her parents and brothers. She loves them. She just finds them kind of annoying, though she’s not above apologizing for her behavior.

Elliott’s characteristics aren’t markers of being a Strong Female Lead so much as just an actual teen girl, the kind you probably know, or maybe were. With this complexity in mind, it makes sense that on Elliott’s 18th birthday, she and her friends Ro (Kerrice Brooks) and Ruthie (Maddie Ziegler) decide, with age-appropriate recklessness, that it’s time to have a transcendent experience. They obtain psychedelic mushrooms and head to a little wooded island to camp out and experience their trips, whatever they might be like. Elliott is at first disappointed that the shrooms don’t seem to have any effect on her, but then the unimaginable occurs: Her older self suddenly appears at the campfire. (You see now where the film’s title comes from.)

Elliott at age 39 (Aubrey Plaza) is a Ph.D. student and, perhaps relatedly, more cynical than she was as a teen. But she seems delighted to meet her younger self, and offers a load of advice, including warnings to stay away from someone named Chad who might turn up soon. And though the mushrooms wear off, the connection between younger and older self outlasts the drugs’ effect, to both Elliotts’ surprise.

This all happens very early in “My Old Ass.” What follows is an idyllic summer of self-realization. Chad (Percy Hynes White) does materialize, as a suspiciously nice college student hired as a summer worker on Elliott’s family farm. Heeding her older self’s exhortations, Elliott makes a point of trying to avoid him, while doing the opposite with her family. Not entirely certain why she’s doing this but trusting her older self must have a reason.

Of course, those of us who were once teenage Elliott’s age understand. The day we turn 18, the idea that we’ll eventually be 39 is so remote that it might as well be fantasy. That’s half the story that Park has written here: a nostalgia-tinged tale about noticing the moment and relishing in it before it becomes a memory. “My Old Ass” has the great advantage of being shot on a pristine Canadian lake in a picture-perfect small town, which gives everything the dappled-light dreaminess of our best childhood summer memories. Even then, though, the best thing about “My Old Ass” is Maisy Stella’s performance, which feels easy and authentic and lived-in. She cut her teeth as a child actress on the ABC series “Nashville,” but her work here suggests the arrival of a talent to watch.

There are, of course, some plot holes — after all, it’s essentially a movie about time travel. Smartly, Park’s screenplay never really tries to explain the mechanics of Elliott’s connection to her older self. Just take it for granted because this movie has a lot to say. In addition to its encouraging note about cherishing the present, the film is full of thoughts about looking toward the past with grief. Elliott’s older self harbors regrets that have turned her from her sunny self to someone considerably more bitter and unhappy. But the film dares us to consider a difficult question: If we knew what would happen in the future, would we change our decisions in the present?

Movies like “Eternal Sunshine of the Spotless Mind” and “Arrival” also deal in those questions, so it’s not untrodden territory. But in “My Old Ass,” it feels fresh, perhaps because it’s wrapped in the trappings of a coming-of-age story.

This is not really a movie for teenagers or, at least, not only for teens. At many points along life’s path, we make decisions that we might later wish we could change. But life’s not a movie, and time moves in one direction. The trick to contentment is learning to live with our whole hearts anyway.

‘MY OLD ASS’

Rating: R

When: Opens today

Where: Wide release

Running time: 84 minutes

3-1/2 stars (out of 4)