I don't think that "the youth" works here. "The youth" can mean one young person (in which case the plural is "the youths") or it can be used in the singular to refer to a group of young people but this is only when you are talking about young people in a general sense e.g. "the youth of today" (which would be ungdomen in Swedish rather than ungdomarna). So for me "The youth sat on the floor" sounds as though one person or possibly (although improbably) as though all young people or a whole generation sat on the floor.
Honestly, this is about the English language. We're here to learn Swedish. Youth was given as a translation of the word when we learned it. If we use it to translate we show thag we know and understand the meaning. This is only annoying when it is not counted as correct and it doesn't teach us anything about Swedish.
I agree. Youth is generally used as a descriptor anyways. For example a soccer league for 9 year olds would be a "youth soccer league." But NOBODY would ever point at the field and say, "the youth (or youths, or youngsters) are on the field." Everyone I know would say "kids." As kids get older, they are referred to as "teens."
Would "kids" have been accepted? In English, it is not actually true that "youngsters" always indicates a group younger than "children." "Kids" would be the most common term to refer to a group of older children. If I saw a group of 10 year olds, I would refer to them as "kids." Never in my life have I ever heard a native English speaker refer to such a group as "youngsters." Referring to such a group as "children" would not be wrong. A group of 10 year olds singing could be referred to as a children's choir.
I'm not quite sure about the extension of 'kids' so I've asked another person on the team. en ungdom is a young person maybe from (a bit older) teenagers up to 25 or so. We're not talking about children in this sentence (except maybe in the legal sense where you're a 'child' until you're 18, but in ordinary speech we don't call older teenagers barn). Jodie Foster in your movie example is ett barn and I wouldn't call her en ungdom. A group of 10 year olds are likewise never referred to as ungdomar. You can call kids in their lower teens ungdomar if you want to flatter them, but barring that I'd usually start at maybe 16 or so (I call the younger ones tonåringar).
Yeah, I responded before I read. "Youngsters" just makes me feel like I'm 80 screaming about kids getting crazy with their rock and roll and hula hoops. :)
Of course, we also don't really have ONE word to describe a person who we live with and are in a relationship with. There's "Live-in boy/girlfriend," I guess, but even then...
There have been several comments to this end, but I'll add another. "Youngsters" is not a good English translation of "ungdomar". If, as stated elsewhere, "ungdomar" range from 16 to 25, it's just not the right word. "Youngsters" could be anywhere from maybe 8 to 20, but only a crotchetly oldster would use the word for anyone over 20. "Youth" isn't really right either. For a group of mixed ages from 16-25, I'd likely use "young people".
I don't think there is a decent approximation into English, really. I would personally consider a wider range than you would for "young people", and I suspect it varies quite a lot between different speakers. All three options should be accepted - and the preferrable solution would have been a feature where the contributors could add explanatory notes to words or sentences. That way, people could have checked the note for ungdomar while doing the lesson.