Here’s the curve (X is age, Y is a measure of one’s happiness)
As you can see, it is lowest for people in their 40s, then starts to improve past that point. To understand more about that and why you need to hang in there if you are in your 40s, read this: The Real Roots of Midlife Crisis in The Atlantic.
Two additional comments:
1) If you are in your 30s, you can expect this to happen, so take stock and think about ways to prepare for it.
2) Obviously this is a large generalization. Still, there is much merit in it, I believe.