From the viewpoint of someone who recently purchased a mare who displayed signs of arthritis on a x-ray, I agree with jaydee. If the mare is only beginning the stages of arthritis, and you put her on a vet recommended supplement and you're willing to put in the extra effort to keep her sound, then go for it!
In my case, the vet said she was just showing signs of arthritis starting.. after she had seven years on the track. She said there was still a lot of cartilage left in the main area of worry. She recommended a supplement (which we have her on now) that would greatly slow the progression of the arthritis and she said that Indie could probably remain sound with a moderate workload, assuming we keep her on the supplements and take care of her legs.. for the next ten years. She said that she personally, has seen many OTTBs with similar injuries be good riding horses into their 20s.
Forever loved, never forgotten; my beautiful Indie. <3 Hoofprints on my heart.