LOOSE ENDS: This week's Billboard points out that five teen-age solo artists have had hits in the past year and a half, the biggest chart showing for teens since the halcyon days of Tony DeFranco, Leif Garrett and the newly resurgent (maybe) Donny Osmond. The latest crop: Janet Jackson, who was 19 at the time of "What Have You Done for Me Lately," current popster Debbie Gibson, Texas guitar whiz Charlie Sexton, balladeer Glenn Medeiros and rapper L.L. Cool J. . . . Last weekend, the Libertarian Party nominated former Texas congressman Ron Paul for president. But before they chose Paul, at least some highly placed Libertarians had their eyes on somebody who's more familiar to rock fans than politicos: Frank Zappa. Robert Murphy, a Libertarian delegate from Oklahoma and a frequent candidate himself, approached Zappa before the party's Seattle convention and asked him to attend the gathering and consider the offer. Zappa has appropriate credentials: He knows his way around Capitol Hill (he's testified in front of Congress lately), he's often sought by television shows (he's been quoted as an expert on political and music-related issues frequently in recent months) and he's had 20 years experience pointing out the foibles and follies of life in the United States. But while he may have made a visible, colorful candidate, Zappa declined to attend the convention--and after studying the party platform and meeting with Murphy for several hours in L.A., he declined to run. The reason? "Philosophical differences," says a Zappa spokesman.