I've had PRP for elbow tendonitis, too late and didn't help (just hurt a lot!). For PRP to have an effect on knees, you probably need multiple injections (my OS doesn't believe the evidence is there for articular cartilage, only soft tissue inflammation in tendons and the like, but he is quite conservative)
I have some cartilage issues in my knee (presumably arthritis is also your issue) and I find hyaluronic acid injections have helped me which are indicated for arthritis, helping inflammation and lubricating the joint. I have been having these every 18 months or so since Dec 2010 (I'm 42 now)
If you look in the 'injections into the joint' section, there are posts there. Also search for posts by MDAL who has had success with PRP but a lot of injections (a dozen or more I think) which depending where you live could get very expensive
Good luck
