To determine whether the age at onset of rheumatoid arthritis (RA) has increased in Japan using a nationwide database (National Database of Rheumatic Diseases by iR-net in Japan, NinJa).
We analyzed the data of RA patients who had been diagnosed with early RA (disease duration < 2 years) and newly registered in 2003, 2008 or 2013.
The numbers of patients who developed RA in 2002-2003, 2007-2008, and 2012-2013 were 536, 812 and 1864, respectively. The mean age at RA onset increased significantly from 55.8 years in 2002-2003 and 57.0 years in 2007-2008 to 59.9 years in 2012-2013. The peak age shifted from the 50-59 years age group in 2002-2003 to the 60-69 years age group in 2012-2013. There was no apparent difference in the age at RA onset between male and female RA patients. Notably, in the period 2002-2003, the prevalence of RA was markedly higher in the age group of 50-59 years, which included the first 'baby boomers', than in the age groups of 30-39 and 40-49 years, even with consideration of the variations in the age composition of the general population.
We have demonstrated that the age at RA onset in Japan has increased significantly over the last decade. This can be attributed to Japan's aging population. In addition, the high prevalence of RA among the first baby boomers suggests that environmental factors might also have contributed to the increase in age at RA onset in Japan.