Supplementary Material for: Is frailty different in younger adults compared to old? Prevalence, characteristics and risk factors of early-life and late-life frailty in samples from Sweden and UK

Introduction
Although frailty is commonly considered as a syndrome of old individuals, recent studies show that it can affect younger adults, too. Whether and how frailty differs in younger adults compared to old is however unknown. To this end, we analyzed the prevalence, characteristics, and risk factors of early-life (aged <65) and late-life (aged ≥65) frailty.

Methods
We analyzed individuals in the UK Biobank (N=405 123) and Swedish Screening Across the Lifespan Twin (SALT; N=43 641) study. Frailty index (FI) scores ≥0.21 were used to demarcate frailty. Characteristics of early-life vs late-life frailty were analyzed by collating the FI items (deficits) into domains and comparing the domain scores between younger and older frail individuals. Logistic regression was used to assess the risk factors of frailty.

Results
The pooled prevalence rates of frailty were 10.3 % (95% CI 2.7-32.7), 14.4 % (95% CI 4.5-37.2), 19.2 % (95% CI 2.5-68.5) in individuals aged ≤55, 55-64, 65-74, respectively. Younger frail adults (aged <65) had higher scores in immunological, mental wellbeing, and pain-related domains, whereas older frail adults (aged ≥65) had higher scores in cardiometabolic, cancer, musculoskeletal, and sensory-related domains. Higher age, female sex, smoking, lower alcohol consumption, lower education, obesity, overweight, low income, and maternal smoking were similarly associated with the risk of early-life and late-life frailty.

Conclusion
Frailty is prevalent also in younger age groups (aged <65), but differs in some of its characteristics from the old. The risk factors of frailty are nevertheless largely similar for early-life and late-life frailty.