In English, we would say "The youth are cheerful." It is very uncommon to refer to a single person as "a/the youth." It seems very unnatural to me, is this acceptable in Polish? If so, perhaps the translation of "youth" is incorrect?
A lot of nouns depicting groups of people are singular in Polish. Plural wouldn't work grammatically here, you'd need to change the subject for example to "młodzi ludzie" = "young people".
It does not matter what the Polish is, the English has to work as English. The fact that the Polish is singular must be apparent from the way the material is presented, not by twisting the English into a place it should not go.