From Carol Graham and Julia Ruiz at Brookings Institution:
…Numerous studies have found recurrent patterns between happiness and life satisfaction (while the terms are often used inter-changeably, the latter is a better-specified question) and important experiences such as employment, marital status, and/or earnings. These, in turn, lead to differences in investment profiles, productivity, voting incentives, and attitudes toward health (Graham, Eggers, and Sukhtankar, 2004; DeNeve and Oswald, 2012; De Neve et al., 2013).
Among these relationships, the one between age and happiness—often referred to as “the U-curve”—is particularly striking due to its consistency across individuals, countries, and cultures (Blanchflower and Oswald, 2007; Steptoe, Deaton and Stone, 2015; Graham and Pettinato, 2002). Happiness declines with age for about two decades from early adulthood up until roughly the middle-age years, and then turns upward and increases with age. Although the exact shape differs across countries, the bottom of the curve (or, the nadir of happiness) ranges from 40 to 60 plus years old. Blanchflower and Oswald (2016) find that some markers of ill-being, such as reported mental health and the use of anti-depressants, meanwhile, display inverse patterns, and turn down (as opposed to up) at roughly the same age range in the U.S. and Britain.
Further research is needed to confirm that the bottom of the U-curve coincides with teenage children 😉