Despite the growing consensus that midlife appears to be a particularly vulnerable life phase for lower mental health and wellbeing, little is known about the potential reasons for this phenomenon or who the individuals at higher risk are. Our study used six waves (2013–2018) of the Swiss Household Panel (n = 5,315), to compare the distribution of mental health and wellbeing, as well as their key correlates, between midlife (40–55 years) and younger adults (25–39 years) in Switzerland. Moreover, using network analysis to investigate interrelationships across life domains, we describe the complex interrelations between multiple domain-specific correlates and indicators of both mental health and wellbeing across the two age groups. Middle-aged (age 40–55) individuals reported lower life satisfaction and joy, as well as higher anger, sadness, and worry than young adults (age 25–39), with the effect sizes reaching up to 0.20 Cohen’s d. They also reported lower social support, relationships satisfaction, health satisfaction, and higher job demands and job insecurity. Relationships satisfaction and social support were the most consistent correlates across all three indicators of wellbeing in both age groups. Health satisfaction was more strongly, and directly, interrelated with energy and optimism in midlife compared with young adulthood (0.21 vs 0.12, p = 0.007). Job demands were more strongly linked with anger and sadness in midlife. The network model helped us to identify correlates or their clusters with direct and strong links to mental health and wellbeing. We hypothesised that health satisfaction, relationships satisfaction, social support, and job demands may help to explain worse mental health and wellbeing in midlife.