There are different types of arthritis.  Most commonly, people deal with osteoarthritis otherwise known as degenerative joint disease.  Rarely people deal with autoimmune issues including rheumatoid arthritis.  Degenerative joint disease is a normal part of aging but some of us wear our joints out faster than others due to traumatic events, excessive body weight, faulty mechanics, or just bad genes.  Rheumatoid arthritis requires more timely assessment by a knee specialist while degenerative osteoarthritis can be seen when the symptoms become enough to warrant it.