to me "the youth" is singular without more context, The youth jumped into the pond. The youth were angry about the curfew. My dictionary say youths is plural and collectively is youth.
I'm with you. The only hint is that in Portuguese almost every word ending with -o is male and ending with -a is female, with a few exceptions. Additionally, most of the words in Portuguese have the same noun as in Spanish. Exceptions are more frequent, though. By the way, I googled about general gender rule in Spanish and found this: http://spanish.about.com/cs/grammar/a/genderreversal.htm. As far as I read, they are also true in Portuguese.