In English, the phrase "the youth" could refer to either a young person (a person of youth), the stage of life wherein one is young, or it could refer to all the young people in general. Does the French phrase "la jeunesse" only refer to the stage of life, or can it also be used to mean "a young person?"
I entered "Les jeunesses" and it marked me wrong. :(
It's the definite article that makes it NOT the abstract stage of life.
One just doesn't say The old age has its compensations, one says Old age has .....
THE youth can really only mean an individual, a young person.
I'm pretty sure jeunesse is the abstract, the state of being young. La jeunesse est difficile parfois. Les jeunes doivent explorer.
In UK English we might refer to a particular young person as 'the youth', but this would only be used of a male, never, I think, of a female. Q: "Which one of that group of people was the culprit?" A: "The youth" - meaning the young man as opposed to the older men or women or young women.
'La jeunesse' doesn't seem to be a possible translation for this use, and Duo doesn't accept 'le jeune homme'. I presume the intended meaning of Duo's 'The youth' is in the sense of 'the youth of the country', 'today's youth' or 'in my youth', in which case they need a clearer sentence for us to translate.