If they
They become more and more inclined to a depression, especially at mature age.
They made achievement of definite purposes sense of the life and therefore when they reach them, the sense disappears.
If they do not manage to achieve the, all of them equally do not see special sense in life.
If your teenager has a similar bent, you serve to him huge service if teach to find sense in various hobbies and other ways of forwarding of free time, and especially if teach to see the huge value of friendship and personal relations.