There was an article posted in some thread in the Agility section about retiring vs not retiring dogs in agility. If I recall correctly, at least for the author of the article, she saw her dogs health deteriorate after retiring them. According to the author, she no longer retires her dogs until the dogs "tell" her to do so. It might be a Trkman article, but I don't exactly remember.
That said- I would absolutely "listen" to the dog. If the dog is refusing to jump- game over. There are just WAY to many jumps in agility... In my opinion, a jump refusal is a pretty clear indication of a problem. (Side note- that's exactly how I confirmed Pimg had hurt her knee- a refusal on an 8" jump from the start line: plain as day.) However, if the dog seems to be jumping fine still, then I'd pay very close attention to the weave poles. I'm thinking that the weave poles (if executed with any kind of speed at all) are probably the most straining on a dog's joints. Sure, the contact performances are jarring, but the weave poles are putting massive twisting pressure on knees, elbows, hips, back, pretty much everything.
I guess in the end, if it were my dog, I'd keep them ultra fit and continue agility as long as my dog wanted. Hey- I might even keep doing agility while avoiding hard obstacles like contacts and weave poles. You know there are options there! I don't believe Jumpers in CPE has weave poles. Tunnelers in NADAC is just tunnels (I think). Snooker (in all venues) is full of choices...
I'd say as long as the dog is still enjoying jumping- then there are a lot of agility options.
C-ATCH Pimg - DOB: 2/06, RATO, UJJ (x2), DJA, HIT, CA, CGC, High in Games, Reserve High in Standard- CPE Nationals 2014
RATCHX High Jinks vom Neuanfang - DOB 9/12 (Gotchya Day: 1/23/2013) CA, agility superstar in training