So, I’m from America. I say, “the teenagers” when I describe like an action or just a description. Like, ‘The Teenagers these days are so rude.’ Or ‘I love the teenagers at St. Als Church.” Saying just teenagers seems more informal but it’s still understood. You can say ‘Teenagers these days are so rude.’ Or ‘I love teenagers at St. Als Church.” Adding “the” sounds more correct but it doesn’t really matter to me at least. I hope that helps.

Yes! But it depends on if you want to be specific. "THE teenagers" refers to a specific group of teenagers. But just "teenagers" refers to all teenagers in general.