Megan Park’s “My Old Ass” is premised on a familiar conversation starter: if you could tell your younger self anything, what would it be? Writer/director Park dreams up a supernatural scenario in which this actually comes true for protagonist Elliott, played at age 18 by Maisy Stella (in her film debut) and at age 39 by Aubrey Plaza.

On her 18th birthday, Elliott, the daughter of Canadian cranberry farmers, ferries herself and her two best friends, Ruthie (Maddie Ziegler) and Ro (Kerrice Brooks), in her motorboat to a small island in Lake Muskoka for a celebratory mushroom trip. While Ro spends the evening dancing and Ruthie communes with the bunnies, Elliott hallucinates her 39-year-old self, or her “old ass,” who imparts a word of advice — spend more time with your family — and a warning: don’t date Chad.

Naturally, Elliott almost immediately runs into a Chad (Percy Hynes White), a summer worker on the cranberry farm whom she meets skinny-dipping. Since Elliott is queer, and has sparked a summer romance with the local barista, Chelsea (Alexandria Rivera), the Chad issue shouldn’t be a problem, except that he keeps turning her head. Thankfully, she has “My Old Ass” saved in her phone, and through texts and phone calls, her future self becomes a guiding lifeline through this transitional torment.

Despite its mystical gimmick, “My Old Ass” is a fairly traditional coming-of-age story. It has the shiny, sparkling look of a glossy TV teen drama, with sweeping drone shots to capture the bucolic beauty of Muskoka Lakes. And, the life moment that Park has focused her screenplay on — the weeks before leaving for college — is well-trodden territory in young adult movies. It does boast an uncommonly strong script for the genre, balancing the sappy and sentimental with a slangy/skater queer cool kid attitude and voice inhabited comfortably by both Stella and Plaza.

It also digs deep into life’s hard-won lessons, and the poignant passage of time, a concept that’s almost impossible to comprehend as a teenager and all-too familiar for those on the brink of 40, looking back on life ruefully — how simple it seems when you’re 18 and how complicated it becomes after a decade or two.
“Are we happy?” Is not the question Elliott should be asking her older self, who struggles to answer that. As it turns out, “happiness” is not the things you have, or have accomplished, but the contentment that you can find in the present moment. In fact, her 39-year-old self doesn’t seem to be thriving. She’s a Ph.D. student who disappears to some kind of meditation retreat invented by Penelope Disick. She drops ominous hints about the state of the world in which she’s currently living.

But as they both come to realize over these few short weeks, warnings to avoid the experiences that might hurt are useless. The potential pain is what makes the beauty starker, and sweeter. The real lesson is to never take anything for granted, to live in a place of gratitude, not in the future or the past, but the present.
These concepts aren’t necessarily earth-shattering, but they are profound. They’re universal, because they’re true, and Park imparts these lessons with sincerity and charm in “My Old Ass,” due in large part to the appeal of her lead, Stella, who cuts a unique and distinctive figure for this kind of film as a tomboyish queer woman, decked out in baggy shorts and Birkenstocks. Her Elliott is earnest even in her sarcastic teenage way.

Though “My Old Ass” seems to want to be more revelatory than it actually is (and maybe that’s just the view from the other side of 40), part of life is the continual cycles of young people learning these lessons over and over. This young person just learns it a little bit early, when she’s still fresh enough to appreciate it in the moment. What a gift from her old ass./Tribune News Service

(“My Old Ass” contains language throughout, drug use and sexual material)

“MY OLD ASS”

Rated R. At the Landmark Kendall Square Cinema and AMC Boston Common

Grade: B-