Arthritis is a very common condition. Experts estimate that one-third of adults under 64 have some form of arthritis. Half of the men and people assigned male at birth older than 65 have arthritis. Two-thirds of women & people assigned female at birth older than 65 have it.It’s hard to estimate how common polyarthritis is because many people don’t know they have it & don’t get it diagnosed. This is especially true because the treatment for one joint with arthritis might help decrease symptoms in other joints that aren’t as painful or stiff.