1710, "garment worn by a woman at night," from night (n. ) + shift (n.2). The meaning "gang of workers employed after dark" is attested from 1839, from shift (n.1).  

1951, "a gang of disorderly young people" [OED], from rat (n.) + pack (n.). In reference to the Hollywood circle around Frank Sinatra, from 1958.

1759, "becoming young, growing young in appearance," from Latin iuvenescentem (nominative iuvenescens), present participle of iuvenescere "to grow into youth, grow young again, regain youth," from iuvenis "young man" (see young (adj.)). Its use in a sense of "immature, undeveloped" (by 1875) is etymologically incorrect.

1968, acronym from fictitious "Youth International Party," modeled on hippie.

On December 31, 1967, Abbie [Hoffman], Jerry [Rubin], Paul Krassner, Dick Gregory, and friends decided to pronounce themselves the Yippies. (The name came first, then the acronym that would satisfy literal-minded reporters: Youth International Party.) [Todd Gitlin, "The Sixties," 1987, p.235]

"youth, lad; boy of the lower orders; personal servant," c. 1300 (early 13c. as a surname), originally also "youth preparing to be a knight" (beneath the rank of a squire), from Old French page "a youth, page, servant" (13c.), possibly via Italian paggio (Barnhart), from Medieval Latin pagius "servant," perhaps ultimately from Greek paidion "boy, lad," diminutive of pais (genitive paidos) "child."

But OED considers this unlikely and, with Century Dictionary, points instead to Littré's suggestion of a source in Latin pagus "countryside," in sense of "boy from the rural regions" (see pagan). Meaning "youth employed as a personal attendant to a person of rank" is first recorded mid-15c.; this was transferred from late 18c. to boys who did personal errands in hotels, clubs, etc., also in U.S. legislatures.