Translation:They are inviting the youth and adults.
I don't know, I think we'd rather stay with the literal translation of the noun 'youth'...
'młodzież' is a collective noun, so even if 'youths' exists and is possible, it doesn't seem to be a translation.
inviting youth implies something other than what this sentence is relaying and should not be a correct translation.
It sounds more like a desire to become young again as opposed to 'young people', as a noun I would say the article the is required but even then would (without identifying WHICH 'youth', e.g. the youth of 'the council estate', 'the Jewish youth', etc.) in contemporary English use, more likely refer to a single person, a young lad, than your wish for it to refer to 'young people'. It sounds too archaic, with or without the article imho.
Frankly, I don't think that any of the Polish sentences with "młodzież" sounds really good to me... so this also translates to English in a not-great way.
Anyway, 'młodzież' for sure is a collective noun, it cannot refer to a single person.
"They invite youth" is simply wrong. "They invite the youth" could be interpreted correctly or incorrectly. "They invite the youths" is correct for the meaning you want, even though it is not a collective noun.
What is strange is that 'youth' as uncountable only works in very specific environments, because, like here, by pairing with another noun it defaults to singular. It is important to understand that EN only works to EN rules. This MUST be 'youths' here.
In English I'd more probably write "They're inviting young and old" - which Duo rejected, perhaps because it's too inexact a translation.
Well, 20-somethings are adults, aren't youth (well, at least not in Polish - Wikipedia says that "młodzież" is between 11-13 to 19-21), and definitely aren't old ;)
I put 'young people and grown ups' and it was rejected. Maybe that isn't American English but it is British English