Teenagers aren't adults yet. They have a lot to learn about life. And many of them think that they already know everything (although I don't believe all of them think that).
As probably any therapist will tell you, teenagers are the hardest to get through to, in general.
But despite that, lots of them can be helped. Many teens will listen to helpful advice given by adults, including teens who are battling mental illness. Lots of them can be reasoned with. Not all of them are super-hard to get through to, in my opinion.
That's why psychotherapy works for many teenagers with depression, anxiety disorders, and self esteem issues.
So don't buy into the idea that you can't help most teens. Many of them can be helped and reasoned with. And many of them really are good kids.