Translation:The young adults are not eating chocolate.
But "the" could mean these particular ones were were talking about, but also it could mean "all", no? Like when someone says, "J'aime le cafe." It could be I like this coffee that you been serving me, or that I like coffee in general. At least that's how I understand it (perhaps it's wrong?) So assuming this is true, I would have said "des jeunes..."
"When there is an indefinite article or partitive article in a negative construction, the article changes to de, meaning "(not) any" eg. J'ai une pomme > Je n'ai pas de pomme." (http://french.about.com/od/grammar/a/adverb_negative.htm)
Put more simply, des, une, du, etc. become de when in negative sentences
In French, the present tense (here, "[ils] mangent / ne mangent pas...") can represent any of the following three English thoughts: 1) "They eat / don't eat..." 2) "They are eating / aren't eating" and 3) "They do eat / don't eat..."
In fact, for the first two English sentences, "Ils/Elles mangent..." is the only way to express them in French without adding more precise content (such as "They're in the middle of eating..." / "... in the process of eating...").
I don't think it's a question of one or the other being a preferred translation in this instance: rather, we can't know which English expression would be preferred because we lack any further context for this sentence. If we had "En général" or "À ce moment," we'd be able to choose an English equivalent with more precision. In this case, though, I'd bet money that Duo just wants to remind you that the present tense can do both the job you assigned it and that other one ("... are not eating..."), so you don't forget.