One of the United Nations' Millennium Development Goals of 2000 was to reduce maternal mortality by 75% in 15 y; however, this challenge was not met by many industrialized countries. As average maternal age continues to rise in these countries, associated potentially life-threatening severe maternal morbidity has been understudied. Our primary objective was to examine the associations between maternal age and severe maternal morbidities. The secondary objective was to compare these associations with those for adverse fetal/infant outcomes. This was a population-based retrospective cohort study, including all singleton births to women residing in Washington State, US, 1 January 2003-31 December 2013 (n = 828,269). We compared age-specific rates of maternal mortality/severe morbidity (e.g., obstetric shock) and adverse fetal/infant outcomes (e.g., perinatal death). Logistic regression was used to adjust for parity, body mass index, assisted conception, and other potential confounders. We compared crude odds ratios (ORs) and adjusted ORs (AORs) and risk differences and their 95% CIs. Severe maternal morbidity was significantly higher among teenage mothers than among those 25-29 y (crude OR = 1.5, 95% CI 1.5-1.6) and increased exponentially with maternal age over 39 y, from OR = 1.2 (95% CI 1.2-1.3) among women aged 35-39 y to OR = 5.4 (95% CI 2.4-12.5) among women aged ≥50 y. The elevated risk of severe morbidity among teen mothers disappeared after adjustment for confounders, except for maternal sepsis (AOR = 1.2, 95% CI 1.1-1.4). Adjusted rates of severe morbidity remained increased among mothers ≥35 y, namely, the rates of amniotic fluid embolism (AOR = 8.0, 95% CI 2.7-23.7) and obstetric shock (AOR = 2.9, 95% CI 1.3-6.6) among mothers ≥40 y, and renal failure (AOR = 15.9, 95% CI 4.8-52.0), complications of obstetric interventions (AOR = 4.7, 95% CI 2.3-9.5), and intensive care unit (ICU) admission (AOR = 4.8, 95% CI 2.0-11.9) among those 45-49 y. The adjusted risk difference in severe maternal morbidity compared to mothers 25-29 y was 0.9% (95% CI 0.7%-1.2%) for mothers 40-44 y, 1.6% (95% CI 0.7%-2.8%) for mothers 45-49 y, and 6.4% for mothers ≥50 y (95% CI 1.7%-18.2%). Similar associations were observed for fetal and infant outcomes; neonatal mortality was elevated in teen mothers (AOR = 1.5, 95% CI 1.2-1.7), while mothers over 29 y had higher risk of stillbirth. The rate of severe maternal morbidity among women over 49 y was higher than the rate of mortality/serious morbidity of their offspring. Despite the large sample size, statistical power was insufficient to examine the association between maternal age and maternal death or very rare severe morbidities. Maternal age-specific incidence of severe morbidity varied by outcome. Older women (≥40 y) had significantly elevated rates of some of the most severe, potentially life-threatening morbidities, including renal failure, shock, acute cardiac morbidity, serious complications of obstetric interventions, and ICU admission. These results should improve counselling to women who contemplate delaying childbirth until their forties and provide useful information to their health care providers. This information is also useful for preventive strategies to lower maternal mortality and severe maternal morbidity in developed countries.