Most kids get told (endlessly) that the world sucks, is going to suck worse for them, and they can't do anything about it. It hard to see a way
not to be depressed in those circumstances.
If its true, and the world is doomed; then they should be depressed. If its not, then what better cover for them to adopt until they get away from the despairing adults around them and can actually begin to act of themselves for themselves.