Youth is the age range when a human is young, or before the person reaches adulthood. The word youth can also be used to describe a group of young people, regardless of age. It is also a common adjective for things that are created specifically for youth.
Something can be in its youth if it is newly created.
A youth is also a teenage boy or young man. Only this last definition has a plural form of youths. All other uses of the word are already plural or a mass noun. Sometimes it can be hard to determine if the term is used correctly and is describing only males, or if the author is using the term erroneously to describe both males and females.
In this day and age, the distinction is probably lost on most readers. One should stick with the group youth or use a synonym such as teenagers or young adults.
Examples
In his youth, Hoyer was a steadfast Browns fan. [New York Times]
A new drug prevention initiative designed to provide youth another way to resist peer pressure to experiment with drugs was launched in Rockcastle County on Tuesday. [Harlan Daily]
With the production and show is in its youth, there were obvious kinks still to be worked out as some shows dragged out for hours while last-minute setup was completed and models were prepped.[Daily Herald]
Police are looking for a youth in a dark hoody who was seen riding away from the area on a push bike. [Spalding Today]
Several incarcerated youths in Utah had the privilege of working with nationally acclaimed muralist Emanuel Martinez. [Deseret News]
The three youths who were arrested for a series of snatching cases in Salt Lake are no less adept than the most professional snatchers. [Times of India]