AALIYAH'S career started and ended with people talking about her age. In 1994, when she was 15, she released her first album, ''Age Ain't Nothing but a Number,'' which sold a million copies. She didn't sing like a little girl -- even then, she had a stronger voice and a more sophisticated approach than most pop singers -- and she didn't act like one, either: the child star was reported to be a child bride, secretly married to her mentor, the R & B crooner R. Kelly. (Neither ever confirmed the marriage.) A week ago Saturday, when her airplane crashed in the Bahamas, killing her and eight other passengers, people were talking about Aaliyah's age again. She was only 22, but she had recorded some of the most innovative and influential pop songs of the last five years.
Aaliyah's second album, in 1996, was called ''One in a Million,'' and it teamed her with the producer Timbaland; his computer-programmed beats fitted perfectly with her cool, breathy voice to create a new kind of electronic music. She sang songs about love, to be sure, but she often sounded as if she were extolling the mathematical perfection of Timbaland's music machines. ''Your love is one in a million, it goes on and on and on,'' she cooed, as if love, too, were nothing but a number.
In 1998, she released the song ''Are You That Somebody,'' which was a musical milestone; the critic Simon Reynolds called it ''the most radical pop single'' of that year. Timbaland created a tricky, stop-start beat that sampled a gurgling baby, and Aaliyah found a way to turn it into a catchy tune. Where most divas insist on being the center of the song, she knew how to disappear into the music, how to match her voice to the bass line -- it was sometimes difficult to tell one from the other. This new approach helped change the way popular music sounds; the twitchy, beat-driven songs of Destiny's Child owe a clear debt to ''Are You That Somebody.''
Two years later, Aaliyah returned with the single ''Try Again,'' which sounded nothing like ''Are You That Somebody'': Timbaland created a fuzzy, booming soundscape influenced by the dance genre known as acid house, and Aaliyah wrapped a serpentine vocal line around the beats. ''Try Again'' helped smuggle the innovative techniques of electronic dance music onto the American pop charts, and it established Aaliyah as pop music's most futuristic star.
So how do you sing a pop song on top of such odd sounds? Aaliyah figured out the answer long before most of her contemporaries: you don't. Many of her songs weren't really songs at all, but simple vocal riffs, repeated and refracted to echo the manipulated loops that create digital rhythm.
''Try Again'' consisted mainly of Aaliyah mouthing a familiar saying over and over again, a kind of musical hypnosis: ''If at first you don't succeed, dust yourself off and try again.'' Her iterative songs evoked the robo-rock group Stereolab or the house music tradition of sampling and manipulating a single sung phrase.
Aaliyah's new album, titled simply ''Aaliyah,'' was released a month ago, and its most remarkable asset is its restraint: whether she's tackling a mid-tempo ballad or a cutting-edge dance track, Aaliyah sings without histrionics. Her current single is a brilliant confection called ''More Than a Woman,'' an uncharacteristically dense arrangement of digital strings, synthetic bass and lissome rhythms in which she makes an odd promise to a lover: ''I'll be more than a woman.'' She sounds like the mirror image of David, the robot-boy in the film ''A.I.'': a real person chasing an ideal of electronic perfection. David lived forever, but Aaliyah barely got started. She was a digital diva who wove a spell with ones and zeroes, and yet it's strange -- sad, and somehow disorienting -- to see her life reduced to a series of numbers. Nineteen seventy-nine. Two thousand one. Twenty-two.
Photo: Aaliyah, who died in a plane crash on Aug. 25, arriving at the Paris Theater in Manhattan several weeks before the accident. (Associated Press)