At what stage does juvenile arthritis turn into regular arthritis?

No particular stage: Some forms of Juvenile arthritis are not continued into adulthood, but Juvenile polyarthritis with rheumatoid factor positive and juvenile sponxyloarthritis, for example, can continue into adulthood.

It depends: Juvenile idiopathic arthritis (JIA) is not one disease, but actually a group of diseases that cause chronic arthritis (joint swelling, stiffness, and pain) in children 16 years of age and under. Some types of JIA resolve completely during childhood, while others persist throughout a patient's lifetime.