This isnt even a settled fact.

For example: https://www.psychologytoday.com/us/basics/happiness/happiness-over-the-lifespan

Argues a few things:

Does happiness decline with age?

People often worry that happiness diminishes with age, but that’s actually not the case. One large study found just a slight overall dip in happiness between age 20 and age 70—on a scale of 1 to 10, average life satisfaction went from 5.8 to 5.4.

What is the happiness curve?

The happiness curve refers to the trajectory that happiness tends to follow as we age. People begin life fairly happy. Around age 18, their happiness begins to decrease, reaching a low point in their 40s. But after age 50, happiness begins to rise again. This U-shaped happiness curve has emerged consistently in large studies of Western societies.

So there is a common curve, but the curve is gradual.

It also notes the likely causes are that financial stability rises mid-life and improves happiness, while free time is at a minimum at the bottom of the curve and reduces happiness.

This has nothing to do with age and everything to do with culture, which can be changed.