At 52 and 53 respectively, Cher and Blondie's Deborah Harry are currently the oldest women to adorn pop radio since rock 'n' roll made an idol of eternal youth more than 40 years ago. Yet, like the agelessness of Oscar Wilde's Dorian Grey, the agelessness of both Cher's and Miss Harry's music has been purchased with a price. In the case of Cher, the price would seem to be artistic maturity. Having conceived of herself long ago as being only as good as her latest hit, she's made a career of discarding musical styles as soon as they stop making cents. The result is music as narrow as the youthful demographic at which it's aimed. Combined with the narrowness of Cher's vocal range, such music takes on a claustrophobic feel. Believe is Cher's biggest selling album in a decade, but whether it's selling because it dispels the claustrophobia or reinforces it is unclear. The overriding style is latter-day disco, the throbbing, electronically enhanced kind that serves as the pulse to metropolitan night life at its most hedonistic. Even the album's most buoyant melodies are shackled to rhythms designed to keep the dance floors frenzied. In small doses, the formula is fun, and fun shouldn't be demeaned. The tissue-thin nature of the lyrics, however, leaves no room for Cher the actress to come to the aid of Cher the singer. Those who'd like to know what she can do with more substantial songs are directed to It's a Man's World, her 1996 album of songs made famous by men that, because it yielded no hits, can now be found in bargain bins. Deborah Harry's case differs from Cher's in several ways. Unlike Cher, who since the 1960s has seldom been out of the limelight, Miss Harry has been in the shadows for some time now. The success of her comeback, therefore, depends to a large extent on her ability to reinvigorate the image with which she was identified in her heyday, that of a new-wave Marilyn Monroe as envisioned by Andy Warhol. In songs such as "Boom Boom in the Zoom Zoom Room" and "Dig Up the Conjo," songs that sound like excerpts from a better-than-average B-movie soundtrack, she recaptures her mystique with only a hint of self-parody, no mean trick coming from a woman old enough to be a grandmother. In fact, with the exception of "Screaming Skin" (in which Miss Harry and her co-lyricist Romy Ashby stretch an epidermal metaphor beyond the breaking point) and the title song (in which Miss Harry engages the rapper Coolio in a rap duel that has all the elegance of a mud-wrestling match), the songs on No Exit are as catchy as anything Blondie recorded during its first go-'round. What's missing is any sense that the group's experience of making music is deeper than it used to be. In other words, while catchy choruses and rhythms abound, the wisdom-the appreciation of mystery-that one would like to think comes with middle age is in short supply.