Strange sentence, I see. But I also see the point that they might want to introduce the plural form of the third declension nominative.
Have you met the parrots yet? And the peacocks....watch out for the weasels...
I agree with all the previous comments! You should consider change this sentence for another more logical and which actually makes sense.
Okay, this has to be the worst duolingo sentence I've seen so far. Move over, "ich renne in die Universität" and "j'aime marcher dans la rue"!
At the very least, it can easily earn you a Darwin Award (much like running into universities :)).
Damn, even here moderators are childish pedantic people that care more the spelling of people rather than their true functions.
"The universities are not young men" is a silly, but grammatically correct sentence, it's teaching a predicate nominative construction. Maybe ask yourself, are universities young men?
I think there is quite a difference between whimsical, silly sentences ("my goat drinks beer"), which I personally find quite fun (although I know that they annoy some people who'd rather learn "something useful"), and ones that baffle the reader. If this sentence contrasted two things that were more in the same category (say "schools are not universities" or "youths are not old people"), it would be a lot easier to understand. Such as it is, it certainly works to show off the predicate nominative -- no problem with that. But without a context -and the reader quite likely still trying to get to grips with basic vocab-, the immediate effect is "huh, did I miss something? I'm sure this has another meaning I'm not getting right now".
That's an answer. Thanks. Yeah, I assumed its purpose was more about phrasal construction than useful sentence, but it still made me confused.