Don’t judge a SoL Theatre production by the age of its cast.

“We’re always getting people who are surprised at the caliber of the performance,” observed Danielle Kopf. “Our audiences come back time and time again because they don’t feel like they’re watching a kids’ show.”

Case in point, “Thoroughly Modern Millie,” which runs Aug. 4-6 at 7 p.m. with 2 p.m. matinees Aug. 6 and 7 at the Thunder River Theatre in Carbondale. Tickets are $15 for adults and $10 for kids.

Aspen high schooler Lilli Smalls, who plays the titular character, is only 16 but already boasts over a decade of acting experience. Her first leading role represents a major milestone as she works toward becoming a professional thespian. It’s a goal that’s likely to take her to a city some day, so she has some sympathy for Millie Dillmount’s journey from a small town to the Big Apple.

Smalls, however, has a couple more years to perfect her craft first, and the perfect venue in which to do so.

“SoL really is the stage of life,” she said. “It’s one of the greatest environments I’ve ever experienced.”

She’s not the only cast member enjoying the experience. Beth Caudill, 15, is rising to the occasion in her role as Mrs. Dorothy.

“It’s the most challenging play I’ve ever done,” she said. “The singing and dancing and acting all together makes it really exciting.”

The challenge is precisely the point. SoL is, first and foremost, about education. It draws from area schools and has trained actors from 6-19.

In addition to the more than a dozen productions it has put on in its four years of operation, the nonprofit also provides camps for new thespians.

“We focus on the experience rather than perfection,” Kopf said. “We want them to have a really grand time and learn while they’re doing it.”

Collaborations with local artists is also helping to take things to the next level, and using Thunder River Theatre space is a nice perk, as well.

“I would love for this to be somewhere we do future productions,” Kopf said.

That probably won’t include the musical adaptation of “Elf,” which is slated for December with auditions in the fall.

Right now, though, the focus is on “Millie,” adapted from the 1967 Academy Award-winning film, and set the height of the Jazz Age in New York City, when “moderns” were bobbing their hair, raising their hemlines, entering the workforce, and rewriting the rules of love.

“This is one of my favorite shows of all time,” Kopf said. “It’s so filled with love and fun.”