Maisy Stella couldn’t have guessed that the thing that would prepare her for her first major film role would be … being a regular teenager.

In 2012, when she was 8, a video of her and her older sister, Lennon, singing Robyn’s “Call Your Girlfriend” took off online — a narrower era of the internet when harmonized covers and cup percussion could charm millions of strangers. The duet, which now has more than 31 million views, landed the pair a segment on “Good Morning America.” Soon after, they were cast in the soapy country music drama, “Nashville.”

But it was a four-year pause from acting Ms. Stella took after six years on “Nashville” to attend high school that laid the groundwork for her return to the screen. Now, she stars in “My Old Ass,” where she plays Elliott, a restless teenager whose mushroom trip conjures her 39-year-old future self (played by Aubrey Plaza). The film, written and directed by Megan Park, plays with the buoyant idealism of youth, face-to-face with the hard-earned wisdom of adulthood.

Ms. Stella’s high school sojourn of backpacks and lockers and prom — all the things she had romanticized from watching movies — turned out to be just the dose of adolescence she needed to play a starry-eyed Elliott.

“My prep for ‘My Old Ass’ was those four years,” Ms. Stella, 20, said. “I can’t imagine how different I would have been in the movie if I had been working consistently since the time I was eight.”