In every generation the Young People are a problem — their weird clothes, their crazy music, their incomprehensible slang. And in every generation the Older People struggle to find some means to cope with them, because they are always seen as a problem.
Today young persons, in a bit of social-science creep into mainstream language, have been converted into a category, youth, rather than individuals, and those who use the category are dropping the plural form of the word.
There is no problem with singular youth as a word referring to that span of life that includes childhood and adolescence.
There is evidently still no problem with referring to a single young person as a youth.
But when social scientists, government bureaucrats, and monkey-see, monkey-do journalists get hold of the mass noun youth, the collective term for all young persons, they stop there and begin to shun the plural.
You can see it in a sentence from the report of a state agency: “While community supervision and treatment is appropriate for the vast majority of youth in Maryland’s juvenile justice system, there are youth who require out-of-home treatment in secure settings.”
You can see it in a news story: “Police Chief Melissa Hyatt mingled with youth holding the protest near Franklin High School.”
Such people are going to talk that way, and write that way, and there is no stopping them. Youths for them has become some kind of scorned usage.
Perhaps those who would occasionally like to refer to young persons as individual human beings rather than a category would consider following the example of Vinny Gambini, the Joe Pesci character in My Cousin Vinny, and call them yoots.