Depends on what kind of arthritis you are talking about. Juvenile Rhuemotoid Arthiritis can start in the very early years almost at birth. While Osteoarthritis, starts later in life. It can start as early as a mild form in the twenties, to much more debilitating as we get older. Rhuemotoid arthritis can also start usually in 40's and getting worse as time goes on. It really depends on the person and the lifestyle they lead as well, genetics also plays a role.