For one thing, the quartet's oldest member is 16-year-old Caitlin Smith, who starts her junior year at Fremont High School this fall. For another, all the singers are young women, and they dress as such when they perform.

"It's not like we're in striped shirts and big moustaches," says Caitlin, who sings baritone.

In the two years they've been singing together, the UnderAge Quartet has made its mark in Sweet Adelines International, performing and competing under the auspices of the organization's Young Women in Harmony program. The quartet placed third at Sweet Adelines' International Education Symposium, held last month in South Carolina. There they competed against 25 other quartets in the age-25-and-under category.

Even with this age limit, UnderAge was the youngest group to compete, and at age 13, lead singer Mia Dessenberger was the youngest performer.

Sweet Adelines singers are judged not only on how well their voices blend but on how well they use movement to explain their songs. To keep their harmonies tight and their choreography smooth, members of the UnderAge Quartet try to rehearse at least once a week.

"It's definitely a lot of hard work," Mia says. "We've become really close."

The group got together after Caitlin and Brittany Gilmor, who sings tenor, met vocal coach Martha Segura at a Sweet Adelines party. After hearing them sing, Segura suggested the girls join her daughter Mary's quartet.

"Me and Caitlin were in another quartet" at the time, says Brittany, 14, a freshman at Kings Academy in Sunnyvale. "We didn't blend."

While the UnderAge Quartet's voices blend, sometimes their personalities don't.

"We're like sisters," says Mary Segura, 14. "We get along, but we fight. Basically, it's teenage drama."

Some of the girls' differences are artistic, although they choose their repertoire by "quartet vote." For their last year's IES performance, which earned them fifth place, they sang a fairly serious set. This year they sang parodies of "The Moment I Saw Your Eyes" and "This Little Piggy."

"We like to do really fun things," Brittany says.

"It's always good to make the audience laugh," Mia agrees.

All four girls have other musical outlets besides Sweet Adelines. Last year Mary was the only freshman in her high school's advanced choir. The other three quartet members take private voice lessons from the same teacher. Brittany sings in her church choir, and Mia and Caitlin have performed with Peninsula Youth Theatre and Children's Musical Theater of San Jose.

Not all UnderAge's performances are competitive. The quartet has performed the national anthem at San Jose Sharks and Earthquakes games and at the Siebold tennis open. They've also sung at each other's schools.

Despite a busy singing schedule, the girls all manage to find time for other activities.

"I play softball and hunt with my dad," Mary says.

"We all do other things, like dance," Brittany says. "We leave time to hang out with friends."

Part of the appeal of performing with Sweet Adelines is that the girls get support from older singers in the organization.

"It's like having 100 moms," Caitlin says.

Martha Segura, herself a member of an award-winning Sweet Adelines quartet, says the girls' voices have developed over time.

"There's a fuller sound from all of them," adds the coach. "Once you've sung with someone for a period of time, it starts to gel and the sound gets stronger."

In October, UnderAge heads to Indianapolis for another competition.