@OP:
What age range are we talking about here? Generally, it is not natural to become unhappy simply because you're getting older. Certain things that happen in your life as you age may play a role in your general unhappiness, but just because you're, say 40, shouldn't mean you're naturally any less happy than you were when you were 10.
If you find that you are continually getting more and more unhappy with no outside influences (i.e., marital problems, money problems, etc.) then you may have depression.
As your brain physically ages, if there becomes a problem with the production of natural serotonin, a key player in your mood, this can cause depression. Which IS a treatable disorder with the use of SSRI (Selective Serotonin Re-uptake Inhibitor) drugs.
Good luck.