YOUNGER THAN YESTERDAY: None of the members of New Edition, MCA's black bubble gum pop sensations, was born before 1967. But that hasn't stopped the band from putting together a new album paying tribute to '50s doo-wop classics. Titled "Under the Blue Moon," it features the group's recent hit rendition of "Earth Angel," as well as such ancient oldies as "Duke of Earl," "A Million to One," "What's Your Name" and "Since I Don't Have You." Produced by Freddie Perren, the record also offers an updated version of "Tears on My Pillow" (with vocal help from Little Anthony himself) and a new tune, "Bring Back Those Memories," which pays homage to the doo-wop era. . . . CBS Records is thinking young, too. It's just released a new record by New Kids on the Block, a baby-face, white bubble-gum funk group. Led by 13-year-old vocalist Joe McIntyre (whose grinning photo on the album jacket reveals a set of teeth covered in braces), the record features a cover of the Delfonics' "Didn't I (Blow Your Mind)" and a ballad called "I Wanna Be Loved by You," where the kids rap about their favorite girls. The group's debut is produced by Maurice Starr, who made his name putting together a group of teen crooners called (you guessed it) New Edition.