Thanks for the responses. After doing some further reading on the subject, I am beginning to think that it is either due to constant wear on the joint (from
frequent jumping up and down) along with a certain amount of loosening of tendons and muscles that goes with aging. I don't think he has injured himself per se, I have never seen him limping. I have been watching him closely to see if I can see any gait changes, if there are any they are very subtle. I have started him on glucosamine-chondroitin supplements, hopefully can ward off arthritis to some degree as he ages.
As far as his daughter is concerned, I will just continue to watch her and I have decided that when I do breed her, I will be careful to make sure the male has extremely tight stifles. She and I do agility together and neither I nor her instructor have ever noticed any abnormal gait.
I found an interesting article yesterday, I will give anyone who might be interested the link
Summary of the Condition Luxating Patellas