Interesting that the translation can be both singular and plural. I tested "our young people are very happy" and was marked correct.
While youth can be the age of a person before becoming an adult, the way it is used here is in reference to a group and is plural. So, "our youth are very happy" should be accepted as correct. Sadly, it is not.
the age of a person before becoming an adult = молодість
young people in general = молодь