Frailty is a complex manifestation of aging and associated with increased risk of mortality and poor health outcomes. However, younger individuals (under 65 years) are less-studied in this respect. Also, the relationship between frailty and cause-specific mortality in community settings is understudied. We used a 42-item Rockwood-based frailty index (FI) in the Swedish Adoption/Twin Study of Aging (n=1477; 623 men, 854 women; aged 29-95 years) and analyzed its association with all-cause and cause-specific mortality in up to 30-years of follow-up. Deaths due to cardiovascular disease (CVD), cancer, dementia and other causes were considered as competing risks. The FI was independently associated with increased risk for all-cause mortality in younger (<65 years; HR per increase in one deficit 1.11, 95%CI 1.07-1.17) and older (≥65 years; HR 1.07, 95%CI 1.04-1.10) women and in younger men (HR 1.05, 95%CI 1.01-1.10). In cause-specific mortality analysis, the FI was strongly predictive of CVD mortality in women (HR per increase in one deficit 1.13, 95%CI 1.09-1.17), whereas in men the risk was restricted to deaths from other causes (HR 1.07, 95%CI 1.01-1.13). In conclusion, the FI is a strong mortality predictor especially among younger individuals and its associations with cause-specific mortality are sex-specific.