Youth orchestras remain eternally young; to be sure, their members grow up and move on, but they are replaced by other young musicians.

Yet such ensembles do grow — if not up, then "out," by expanding their programs and touching the lives of an increasing number of kids.

For example, Boulder's High Plains Youth Symphony, founded only a year ago, launches it second season Sunday with the debut of the 25-member Sinfonietta, a second ensemble designed to give still younger players orchestral experience.

"It's a training orchestra for string players," says Amy Andersson, music director for High Plains. Its members range in age from 9 to 11 and in the ensemble they learn teamwork and repertory.

"And, of course, it's a feeder orchestra for the more mature symphony."

To give the kids a sense of their own identity the Sinfonietta has its own conductor, Emily Bowman, a Longmont resident with wide orchestral experience.

"They rehearse right before High Plains," Andersson says, "and many of them stay on to listen to the older ensemble at work.

"It's an ideal experience for budding string players."

Bowman and the Sinfonietta open Sunday's program at 2:30 p.m., after which Andersson and High Plains take the stage.

Soloist with the Symphony is violinist Judith Ingolfsson, who joined the University of Colorado string faculty a year ago.

Having won the Gold Medal at the 1998 Indianapolis International Violin Competition, Ingolfsson now enjoys an international reputation as a phenomenal musician and virtuoso.

"Judith and I have mutual friends in Berlin," Andersson says. "I met her after a recital at CU last year and asked whether she would be interested in performing with High Plains.

"She views her appearance with us as part of the university's public outreach program."

On Sunday Ingolffson will perform Haydn's C Major Concerto.

"It was her choice," Andersson says. "She had never played the work before and was dying to do it."

Haydn, born in 1732, wrote the concerto right after his appointment at the Esterhazy court.

"He was just 33, and this was a major turning point in his life," Andersson says. "You can feel his youth in the score; it's full of life and energy."

The concerto calls for only string accompaniment.

To make the concert a celebration of Viennese classicism, Andersson decided to "frame" Haydn with two works by Mozart: the overture to "The Magic Flute" and the final movement from the "Jupiter" Symphony, the last of his 41 scores in that genre.

"The Overture exposes orchestra members to opera, and gives me a chance to talk to them about it," Andersson says. "And the great double fugue that concludes the 'Jupiter' gives them a feeling for the fusion of Bach's counterpoint with Mozart's classic structure."

Returning to the concept of growth, Andersson points to events that expand High Plains' activities in this second season.

"We're having our first youth competition this week," she says. "The winner will perform with the symphony on Feb. 3."

Andersson has arranged two "side-by-side" appearances for the orchestra. It will share the Macky stage with the Boulder Phil at the orchestra's annual family concert in January, and in June it will join the Colorado Music Festival in its Young People's Concert, the opening event of that season.

Andersson will again conduct the CMF orchestra for the performance.

And in May, High Plains will host a second CU soloist, pianist David Korevaar, who will play Mozart's B Flat Concerto, K. 595, the composer's 27th — and last — work for keyboard and orchestra.