Gender also plays an important role. Before age 45, osteoarthritis occurs more frequently in males. After age 55, it develops more often in females. The cause for this gender difference in the higher rate of arthritic knees in women may be high-heeled shoes. The younger males developing OA has traditionally been blamed on high impact sports, if this is the case than those statistics should be changing soon.
Females seem to be playing high impact sports just as often as males now a days.
Obesity is another factor. Knees, for example, sustain an impact three to five times the body weight when descending stairs. If you weigh 200 pounds, that is 600-1000 pounds banging on those knees. If you are obese, any loss of weight will be beneficial. A loss of only five pounds can eliminate at 15-25 pounds of stressful impact on the knees.