The inspired brainchild of the songwriters Robert Lopez and Jeff Marx, this canny toy chest of a musical takes its stylistic cues from "Sesame Street," from its cheery urban set to its singing puppets of assorted colors and dispositions. And in doing so it becomes the first mainstream musical since "Rent" to coo with such seductive directness to theatergoers on the fair side of 40 in their own language, in which irony is less a mind-set than a loosely worn style. Directed by Jason Moore, with a book by Jeff Whitty, the show applies the coaxing, learning-is-fun attitude of children's educational television to the R-rated situations of postcollegiate life in the big city. Featuring a pitch-perfect ensemble of live performers and oversize hand puppets, "Avenue Q" is to "Sesame Street" what Mel Brooks's "Producers" is to vintage Broadway musicals: a connoisseur's tribute to what it only seems to send up.