There is the word “youth” which means “the condition of being young, or the appearance, freshness, vigor, spirit, etc., characteristic of the young. As a ‘condition or state’, this doesn’t really have a plural. The same form is used with plural determiners. “His youth was apparent in his face”, “Their youth showed in their idealism.”
There is also a word “youth” meaning “young people in general”. This is a collective noun and so is generally already plural. “The youth of today have advantages which older people didn’t have.”
There is also a word “youth” meaning “a young person, especially a young man”. This has the plural form “youths”. “Three youths were arrested for causing a disturbance.”