Historically, happiness is highest among the young & old, dipping in middle age. One of the most robust findings in this literature - even replicating in chimps!
Young people (esp 18-25 y/o women) are much more unhappy today, breaking this relationship for the first time.
since this is getting traction, the most important thing here is that while gdp and happiness/life satisfaction are strongly correlated over time, there appears to be, especially in high income countries, a decoupling AND recent downward trend