In their "youngdom." Love it! It seems as tho initial O's picked up tails during their trip into English, becoming Y's, much like initial i's picked up tails becoming J's.
Just adding that the accepted versions apart from their are his/her/one's/your/its.
Your is accepted here if you mean it in the general sense = if you could have replaced it by one.
"His" should work, but you can't use "Sin" in place of "Min/Vår" or "Din/Er"
Is "sin" used because the gender is unknown? I mean, could we say "i hans ungdom" or "i hennes ungdom"?
I think it would depend on how its used. Sin is used when the possessor is the same as the subject of the sentence, e.g., "She kissed her husband" meaning her own husband would use sin, but if it were someone else's husband it would be hennes. So, I think the way it would be used (please someone correct me if I'm wrong here!) would be in a sentence like "In his/her youth, he/she got into trouble a lot", but I think you'd use hennes/hans in a sentence like "I knew him in his youth"?
2 rules. 1st as cvictoria42 said: if the object is possessed by the subject then you use sin t.e. Han tvättar sina händer. Otherwise use hans: Han hade på sig hans skor. (they belong to another person). 2nd rule: sin never as an subject: Måns och hans fru...
No, the plural forms is used to own a plural noun, regardless of the number of the third person pronoun. Ungdom is singular uncountable here, so it has to be sin.
Is this a common way to begin a past-tense sentence, like "Als ich ein Kind war" or "Toen ik een kind was"? Like "I min ungdom har jag spelat fotboll" or something? (I am just guessing that is correct past tense word order since I have not learned it yet...)
Yes, pretty much. I sin ungdom brukade hon spela fotboll 'In her youth, she used to play football/soccer'. The tense in your sentence must be the simple past spelade, not the past perfect that you used, because the past perfect refers to a result, and we don't have that here. I min ungdom spelade jag fotboll.
Edit: I think I sort of missed the point here at first. You should say När jag var barn… if you're speaking about your childhood. När jag var ung… is about when you were young, not when you were a child.
so youth here means at a time someone was young, but what's the word for youth, as in young people, "today's youth"
that is only for "their", "sin" refers to the subject and could be anything: his, her, it's, their...