Some of us find it hard to grow up and then some of us never do. The Everteens have found a way to stay young forever -- as teen-agers of the '50s. The National Theatre's "Monday Night at The National" tomorrow features this five-member a cappella group in two free performances at 7 and 8:30 p.m.

When they are not working for the Environmental Protection Agency, the Defense Department or the Smithsonian Institution, members of the Everteens are recapturing the street-corner harmonies made famous by the Moonglows, the Shirelles and the Big Bopper.

"We're 'kids' who spent our teens in the '50s," says tenor Bill Carroll. "The music we grew up with has such a strong hold on me. I've been doing it for almost 30 years."

Featured last week on "America Today," the United States Information Agency's version of "PM Magazine," the Everteens hosted a '50s prom -- just one of more than 35 appearances the group makes each year. Members have perfected 48 songs, up from just eight several years ago when the group made its debut at a friend's housewarming.

As the group's popularity has increased, so has interest in joining it, but Carroll says, "most of the groups back then had four or five people in them. If there were more voices in our group, we'd lose some of the street corner/doo-wop sound."