The researchers have analyzed data collected from 1,546 people aged from 21 to 99 in San Diego, USA. People in this study had to fill out a long survey including questions about their physical, cognitive and mental health. Survey asked how happy and satisfied with their life they were, and how depressed, anxious or stressed they were.
The results of the study demonstrated that old people were more disabled physically and had more cognitive impairment than younger adults, but their mental health was actually better. Younger adults in 20s and 30s reported the highest levels of depression, anxiety, and stress. Also, they showed the lowest levels of happiness, satisfaction with life, and wellbeing. And older people were the happiest.
More details here.