MOST 13-year-olds avoid the tag "immature" like the plague, but Marques "Batman" Houston, Jerome "Romeo" Jones and Kelton "LDB" Kessee have adopted the word as the name for their soul-harmony trio. In fact, the youthfulness of these three 13-year-olds from Los Angeles is a large part of their appeal, and the latest Immature album, "Playtyme Is Over" (MCA), has already yielded two gold singles, "Never Lie" and "Constantly."

"When we say we're Immature," Houston explains, "we don't mean we're bad kids. We just mean we're young and like to have fun. If you compare us to Jodeci, we don't sing about bumping and grinding and all that nasty stuff; we sing about thinking about a girl and giving her gifts and missing her, like a young person would. We've never had serious, serious relationships -- more like friendships. But we are looking for girlfriends."

It may be difficult for Houston and his mates to preserve that youthful innocence in the face of their snowballing success. Their hit singles have created a demand for personal appearances, and the three youngsters have left the eighth grade and hired private tutors while they criss-cross the country. Immature headlines at Constitution Hall Saturday as part of their current tour. Between concerts, interviews, recording, rehearsals and an appearance in the movie "House Party 3," there's not much time for a normal adolescence.

"You have to have a bit of an adult mind for this business," Houston admits, "but you don't have to go overboard and try to be an adult. People won't like it. If I were an adult, and I saw a little kid trying to be an adult, cursing and acting nasty, I wouldn't like it. So we try to be nice to everybody. We try to stay young and enjoy our childhood while we can. If you grow up too fast, you'll miss out on playing sports, going to school and having fun with your friends. You'll end up missing the childhood you never had."

"It's hard not to grow up fast in this industry," agrees Kessee, "because you have to be mature for your age to handle it. But there's a time to be serious and a time to play. We just try to be ourselves, and if we forget how, we still have our families and old friends to remind us."

The group got its start when Chris Stokes, an enterprising young songwriter-producer from L.A., started auditioning local youngsters for a preteen singing group he had envisioned. Stokes wanted a group that would combine the hip-hop harmony singing of Boyz II Men with the innocence of the early New Edition.

"I met Chris when I auditioned for him back in '89," Jones recalls. "I got the gig, and we started looking for other members. I was just going past the Community Elementary School one day when I heard some music. It was a talent show, and I saw Batman on stage dancing and I said, Hey, maybe he could be in the group.' He tried out for Chris and me, and we said, That's cool.' We had another member named Half Pint, but things didn't work out with him. Instead of trying to find another singer, we just brought LDB, who was already a drummer with the group, up to the front."

Before Kessee became a full-fledged member, though, Immature released a 1992 album, "On Our Worst Behavior" (Virgin), which was a complete flop. It took them two years to regroup with a new label, a new lineup, a new look and a new sound.

"The first album was more bubble-gum, more kiddy-like," says Jones. "That was something both the group and management felt had to change. We toned down our looks; now we don't have the big hats or colorful clothes. And we changed our sound so it was more like Jodeci or R. Kelly, something both kids and adults could relate to."

Although Immature sprinkles some raps throughout "Playtyme Is Over," the trio emphasizes their sweet vocals. It's a wise move, for Houston is a marvelous ballad singer with a creamy tone and an appealing sincerity, and his two partners cushion his voice with smooth harmonies.

"We like a wide variety of music," Jones points out, "rap, dance and R&B. But when it came to Immature, we wanted to do what we do best, and that's singing. We can rap good, but not like we can sing. Being young we can hit those high notes that the older artists can't touch. But those low notes; they're rough." IMMATURE -- Appearing Sunday at DAR Constitution Hall with Monteco, Vicious and Visions. To hear a free Sound Bite from "Playtyme Is Over," call 202/334-9000 and press 8101. CAPTION: From preteens to teen dreams: (from left) LDB, Batman and Romeo.