Our study examines the contribution of genetic and environmental factors (both shared and unique) to frailty, measured using the Rockwood Frailty Index (FI) in a sample of twins from the St Thomas’ UK Adult Twin Registry. The FI was based on 39 items of potential health deficit. Study participants were 3,375 volunteer adult twins (840 monozygotic and 802 dizygotic twin-pairs) 40.0–84.5 years old. First, we used structural equation modeling to estimate the relative contribution of genetics and of the shared and unique environment to variance in FI adjusted for age. In a second analysis, multiple linear regression was used to examine variance in FI as a function of father's occupational class (a component of shared environment and a measure of childhood socioeconomic status [SES]), adjusting for age, birth weight, marital status, and health behaviors (smoking, alcohol consumption, and physical activity). Statistical analyses were conducted using IBM SPSS^® Version 22 software and Mx open source software. Findings showed that 45% (95% confidence intervals [CIs] 30–53%) of the inter-individual variation in FI was heritable and 52% (95% CIs 47–57%) was due to the individual's unique environment. Multiple linear regression also showed a small but statistically significant inverse association between father's occupational class and FI, mediated by one's own educational attainment and birth weight. Our results indicate that frailty is both genetically and environmentally determined. Thus, its prevention and management call for a multifaceted approach that includes addressing deleterious environmental factors, some of which, like childhood SES, may act across the life course.