juvenescent (adj.) Look up juvenescent at Dictionary.com
1759, 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.)). Use in a sense of "immature, undeveloped" is etymologically incorrect.
juvenile (adj.) Look up juvenile at Dictionary.com
1620s, "young, youthful," from Latin iuvenilis "of or belonging to youth, youthful," from iuvenis "young man, one in the flower of his age" (in Roman use, the period just beyond adolescence, from age 21 or 25 to 40), noun use of an adjective meaning "young" (source also of French jeune; see young (adj.)).

Meaning "pertaining to or suited to youth" is from 1660s. As a noun, "a young person," from 1733. Juvenile delinquency first recorded 1816; Juvenile delinquent the following year. Slang shortening juvie/juvey is recorded from 1941 as "juvenile delinquent," 1967 as "juvenile detention."
juvenilia (n.) Look up juvenilia at Dictionary.com
"works of a person's youth," 1620s, from Latin iuvenilia, neuter plural of iuvenilis "of or belonging to youth" (see juvenile).
juvenility (n.) Look up juvenility at Dictionary.com
1620s, "state of being young or youthful," from Latin iuvenilitas "youth," abstract noun from iuvenilis "of youth," from iuvenis "young man" (see young (adj.)). Meaning "anything characteristic of youth" is from 1660s; that of "juveniles collectively" is from 1823.
Juventus Look up Juventus at Dictionary.com
Roman god of youth, personification of iuventas "youth, young person," originally "the age of youth" (from 20 to 40), from iuvenis "young man" (see young (adj.)).
juxtapose (v.) Look up juxtapose at Dictionary.com
"to place (two or more objects) side by side or close together," 1826, a back-formation from juxtaposition or else from French juxtaposer (18c.). Related: Juxtaposed; juxtaposing.
juxtaposition (n.) Look up juxtaposition at Dictionary.com
1660s, from French juxtaposition (17c.), from Latin iuxta "beside, very near, close to, near at hand" + French position (see position (n.)). Latin iuxta is a contraction of *iugista (adv.), superlative of adjective *iugos "closely connected," from PIE root *yeug- "to join."
jynx (n.) Look up jynx at Dictionary.com
"wryneck," 1640s, from Modern Latin jynx (plural jynges), from Latin iynx (see jinx). As "a charm or spell," 1690s.