The youth mainly means the state of being young. "In my youth I did silly things." "Youth is an innocent time of life."
It is true that it can also mean young people in general. (Not sure about 'one young person'... "I gave the letter to the youth" sounds quite weird.).
Anyway, I don't know how to express the state of being young in Indonesian (is it kemudaan, pemudaan...?), so I cannot really report this.
Sorry, Utnapis, but LilinKecil is right. "The youth" can mean just one young person, although it's a pretty old-fashioned usage. The sentence you made up could well be found in a line from Shakespeare :)
When we mean the state of being young, we use "youth" without the definite article "the," as you wrote in your first two examples.
In another question, Duo gives the translation "zaman muda," but Google agrees it can be "kemudaan," so one of your guesses is correct.
Thank you for clarifying these points. It is nice to have the definite meaning from a native speaker.
Now that my confidence in my English intuitions are shaken a bit, I began to wonder about another thing about this question. The default translation of "para anak muda" would seem to straightforwardly translate as "young children". Maybe "anak muda" can somehow also mean just a young person, I don't know. But assuming it just means " a young child", then the translation doesn't really match?
Oh, I didn't mean to make you doubt so much! As LilinKecil said, it can have both meanings, so Duo should accept both translations. But my guess is that they meant the collective noun, since the singular is, as I said, used more in literature than in general.
In the translation, "anak muda" does mean one young person, but the word "para" makes most singular nouns plural or, in this case, collective. I hope this fully answers your remaining questions. It's nice to have this kind of dialogue here!