I am surprised to see that nowhere in this discussion does anyone talk about a corrective osteotomy. It is a procedure done by orthopedic surgeons (all over the US). First you need to identify if your knock knees (genu valgum) cause to pain. Also, try doing some research on this yourself, except try using the term 'genu valgum' rather than 'knock knees'. Yes, genu valgum will most likely result in early arthritis.
I am 22 years old, I have knock knees also. I used to be quite overweight, and two years ago I lost a significant amount of weight. I have always had slight knock knees, but it was never very severe, and never bothered me. I exercise on a regular basis and last summer I tried running. I did this for several months, but stopped as I noticed my knock knees getting worse (appearance-wise). Since then, the knock knees have progressively gotten worse, and starting about three months ago I have started having pain in my legs. I have grown to be quite embarrassed of them myself.
I have been to a handful of doctors who don't appear to believe me that the Q-angles (degree of genu valgum, or knock-kneeedness) is getting worse. I have been doing physical therapy for about a months, but I don't think it is working. My next step is a chiropractor, and then maybe a second orthopedic doctor. I am currently doing my own research on genu valgum, and osteotomies. I suggest you all who are concerned do the same.
Hope this helps some of you!