He's saying: They are aware of stories of problems adults face.

It is absolutely true that someone else's life doesn't define their own future path, but that doesn't mean they don't latch onto those stories and project the negativity onto themselves. One cannot see the future, so it is quite common to look to others to predict what the future may hold. As you point out, the future is often completely different to what you imagined, but it is what you are imagining that shapes how you feel.