You are right, mostly. IN most usages it's a collective noun.
IT also sounds ridiculous unless you are referring to a gang, or youth up to no good.
But people think anyway should have an "s" on the end.... Banging my head into the wall.
Vivek Raman, the word נער is in the Old Testament and means a youth, young man or a servant. In 1 Kings 3, the LORD says to Solomon “Ask what I shall give you” and Solomon in his answer calls himself נער, which translates in the ESV as “a little child”.
The fact that נער is translated in English as “teenager” is probably what makes you think that it’s a modern word.
And as for what age you begin and stop being נער (young), I would guess that it’s as nebulous in Hebrew as it is in English.