PHIt depends which word “youth” we mean.
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.”
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.”