Michael Jackson was just 11 years old when he recorded "I Want You Back" with his brothers in the Jackson Five. The single became number one in 1969, thanks largely to the irrepressible, childish excitement in Jackson's boy soprano. Over the years, Jackson's range has dropped and his songs have grown more sophisticated, but he's never lost the giddy optimism of an 11-year-old who believes the main purpose in the world is fun. That giddiness is the secret ingredient that makes Jackson's "Thriller" (Epic, QE 38112) this year's best serving of adolescent pop.

Adolescent themes dominate the album. The title cut is about horror films ("There's no escapin' the jaws of the alien this time") and the chance to cuddle up with a scared girlfriend. "Beat It" describes the brush-off that every young kid hates. The vocal tug-of-war between Jackson and Paul McCartney over a common girlfriend on "The Girl Is Mine" is full of innocence; there's no hint that she's sleeping with either of them.

While Jackson has retained an authentic adolescent spirit, his music has continued to grow ever more sophisticated. In fact, his new musical maturity only makes his youthful ebullience all the more convincing. His funky, stuttering syncopation on "Wanna Be Startin' Something" propels his brash warning to the gossips who "got my baby cryin'." Jackson's own multitracked vocals and horn arrangements build the song to a feverish delirium. On the other hand, his carefully understated voice and softly echoing harmonies on "Human Nature" capture the weary loneliness of a young man taking a solitary midnight walk.

Jackson gets some big help from selected guests. Eddie Van Halen, the ultimate adolescent guitar hero, bolsters the tension and resentment of "Beat It" with a buzzing solo. It's impressive that Jackson can still sound young and innocent at 24; even more impressive is thatPaul McCartney sounds that way at 40 on "The Girl Is Mine." With Jackson singing high and McCartney singing low, these two grand pop voices blend luxuriously and engage in some funny one-upmanship at the close of this hit single. Vincent Price, star of many late night TV horror flicks, adds an arch vampire rap and his classic evil laugh to the end of "Thriller." The only siblings to appear on Jackson's latest solo album are his two sisters, Latoya and Janet, who each play the title role in "P.Y.T. (Pretty Young Thing)."

The new album was made basically by the same team that made Jackson's fabulously successful "Off The Wall," released in 1979. Quincy Jones produced; Jackson and Rod Temperton wrote most of the songs; Steve Porcaro, Greg Phillinganes and Louis Johnson remain the core players. The disco rhythms of 1979 have been replaced by the pop-soul crossover arrangements of 1982. Jackson has assumed a larger role: He wrote four of the nine songs and coproduced three of them. This team has once again crafted a sound so crisp, melodies so catchy and rhythms so danceable that it hardly matters what the songs are about.

Yet there are signs that Jackson is maturing. "Beat It" concerns the pains of growing up ("Don't wanna be a boy; you wanna be a man"). "Billie Jean" is the angry denial of a paternity claim. "The Lady in My Life" is a sensual ballad with a more carnal expression of romance. While "The Girl is Mine" may be the album's most entertaining cut, "Human Nature" is the most impressive achievement. Written, arranged and played by members of Toto, the song sustains an atmosphere of late-night loneliness. Jackson's slow, whispered vocal swirls evocatively across the nervously clicking rhythm track.

Jackson resorts to unabashed juvenilia on "E.T., The Extra-Terrestrial: The Storybook Album" (MCA, 70000). Although CBS has enjoined MCA from further distribution of this boxed set, there are almost half a million sets on the market that are not affected by the suit filed over Michael Jackson's participation while under exclusive contract to CBS. The set includes a fold-out poster of Jackson and E.T., a lavishly illustrated 24-page booklet and a single album record. The album opens and closes with "Someone in the Dark," a tear-jerker by Alan and Marilyn Bergman and Rod Temperton, which is sung with unrelieved sentimentality by Jackson. The rest of the album has Jackson narrating the tale of the film, with snatches of dialogue, sound effects and the John Williams score interspersed.

Unfortunately, the plot is condensed so much that the narration becomes more of a synopsis of the film than a re-creation. Moreover, Jackson delivers every line with such breathless excitement that there's little difference between build-ups and climaxes. This may be a great gift for anyone under 10, but probably will prove too much to swallow for anyone older.

Michael Jackson's sister has just released a very impressive debut album, "Janet Jackson" (A&M, SP-6-4907). Side one was written and produced by Rene Moore and Angela Winbush with Rufus supplying support. Side two was produced by Foster Sylver with the Sylvers' studio gang helping out. All eight songs are catchy, and Janet Jackson has a high-pitched, giddy voice not unlike her brother Michael's.

On Moore and Winbush's "Say You Do," she slices through the chunky guitars and spunky horns like a siren: "If you want me, say you do!" She halts a moment at the comma and then lets the title line go in a falsetto squeal with all the eagerness of a young romantic unburned by love. With this siren quality, Janet Jackson's debut should go much further than the ill-fated solo records by her sister Latoya.