"Jung" (young) is the basic form of the adjective. You use it when the adjective follows the verbs "sein" (to be) or "werden" (to become). Ex.: "Der Mann ist jung." (The man is young)

When the adjective is placed before a noun, the form changes and an ending has to be added. Ex. "der junge Mann" (the young man). There are several different endings for adjectives before nouns. One of them is "-en" as in "jungen". "Jungen" (and other adjectives with the ending "-en") is used:

1. after the definite or the indefinite article: for all plural forms + for dative/genitive singular (all genders) + for accusative singular before masculine nouns

2. for nouns with no article: for the dative plural + before neutral and masculine nouns in the genitive singular + before masculine nouns in the accusative singular

For more information, see the chapter on adjectives here: http://coerll.utexas.edu/gg/gr/index.html?si=adj