Arthritis can start at any age.
I suffer with osteoarthritis. Although they say this is due to wear and tear and more likely in older people. It actually starts very slowly and gradually and progressively gets worse.
The most common forms of arthritis is Rheumatoid arthrtiis which an autoimmune disorder, and Ostearthritis with loss of cartligate in between the joints causing wear and tear on the joints.
The usual symptoms are pain, sometimes swelling and tenderness of the joints, and stiffness. It is important to keep mobile in between the severe episodes so as not to seize up completely.
Gout and lupus is another form of arthritis and there are other forms too which I am not too familar with.
Hope this answers your question
I'm not sure I understand your question, but if you mean what age, it can start at any age.
The autoimmune type of arthritis can start in infancy; my mother was just diagnosed at age 79.
The wear and tear type of arthritis, osteoarthritis, is associated with old age but again, if someone is born with a joint deformity or suffers a joint injury at a young age, osteoarthritis can set in very early - teens, young adult, etc.
Does that help?