Not for everyone, and not in every society, but strong enough to be a helpful pattern, we see life satisfaction bottoming out in the late 40s for many people. And then it gets better.
There is much in here, including a whole section on brain development.
I found this to be the most helpful idea:
“This finding,” [Princeton researcher Hannes] Schwandt writes, “supports the hypothesis that the age U-shape in life satisfaction is driven by unmet aspirations that are painfully felt during midlife but beneficially abandoned and felt with less regret during old age.”The curve below was compiled by economic Carol Graham and colleagues, looking at averages in many different countries. The X-axis measures life satisfaction on a ten-point scale; the Y measures age.