Objective Age at rheumatoid arthritis (RA) onset varies by geographical latitude. We have investigated to what extent differences in patient-specific factors and country-level socioeconomic indicators explain this variability.

Methods Patients with RA from the worldwide METEOR registry were included. Bayesian multilevel structural equation models were used to study the relationship between the absolute value of (hospital) geographical latitude and age at diagnosis (as a proxy for age at RA onset). We examined to what extent this effect is mediated by individual patient characteristics and by country-specific socioeconomic indicators and disentangled whether the observed effects occurred at the patient, hospital, or country levels.

Results We included 37 981 patients from 93 hospitals in 17 geographically widespread countries. Mean age at diagnosis per country ranged from 39 (Iran) to 55 (Netherlands) years. Per degree increase in country latitude (between 9.9° and 55.8°), mean age at diagnosis increased by 0.23 years (95% credibility interval: 0.095 to 0.38) (reflecting >10 years difference in age at RA onset). For hospitals within a country, this latitude effect was negligible. Inclusion of patient-specific factors (eg, gender, anticitrullinated protein antibodies status) in the model augmented the main effect from 0.23 to 0.36 years. Inclusion of country-level socioeconomic indicators (eg, gross domestic product per capita) in the model almost effaced the main effect (from 0.23 to 0.051 (−0.37 to 0.38)).

Conclusions Patients living closer to the equator get RA at a younger age. This latitude gradient was not explained by individual patient characteristics, but rather by countries’ socioeconomic status, providing a direct link between countries’ level of welfare and the clinical onset of RA.