"I sound like I'm 40 right now," Grace VanderWaal jokes while on the phone with Teen Vogue. In reality, the America's Got Talent winner turned rising pop singer is a cool 15 — but you might not know it from the way she talks about Stargirl, heroine of the Jerry Spinelli novel of the same name, and who she's playing in the upcoming Disney+ film.

She's wrapped filming on the project, which is set to come out in 2020, but the plucky, ukelele-wielding, norm-defying Stargirl has stuck with her even though Grace's work is done. "She is so unapologetically herself and it's ridiculous because she's so young," Grace says, before dropping the 40-year-old remark. "Stargirl is not afraid of anyone, of doing her own thing. She understands the concept that no one cares — or at least people don't care as much as you think they do."

But what Grace admires about Stargirl is also part of what fans admire about Grace herself: her willingness to forge her own path, her self-possession. As she's grown up and away from her AGT days, she's continued to make music on her own terms, album cycles be damned.

Her latest work is the single "Waste My Time," which she wrote, and its accompanying music video, which she came up with the concept for. The song is a synthy dance-pop smash that feels like the next iteration of Grace's musical persona, and it only makes us more excited for what she's working on next (and her upcoming Ur So Beautiful tour, which starts this month).

Teen Vogue chatted with Grace about her Stargirl acting role, her past selves, and her new song "Waste My Time," which came out Aug. 9.

TV: What were you thinking about while writing "Waste My Time"?

Grace VanderWaal: What I like about it is that it has a really nice contrast of kind of messed up lyrics, but it's fun at the same time. That's a feeling I really wanted to portray. Like when things are messed up, people say you shouldn't waste your time, you only have one shot. You're only young once. This is a song [that says], whatever, I don't care, I'll waste it.

TV: You released the song "Ur So Beautiful" in June. How are these songs teasing your next Grace era?