Hmmmm...another good question and I'm not sure there's a "right" answer!
Personally, I think that it depends on the horse and that the horse will let you know when they are tired and ready to retire to the sedate life. Obviously, he's doing well and you're still enjoying him and no doubt he's enjoying life because of that! Some horses have this awesome work ethic and personality that reults in a very enjoyable horse that lives to go places. Other horses are indeed content to hang out with their barn buddies. I've known horses who truly were not happy "retiring".
I say as long as he's comfortable being ridden, especially if he enjoys it, then keep riding. There will no doubt come a time when the rides are just leisurely strolls and no jumping, or fast paces, but those can be the best of times.
Add to that, 18's not really that old :) Where I went to school, we had a 22 year old who was one of our best beginner rider horses and taught many a student how to handle a jump course. He was an x-steeplechaser/foxhunter and he LOVED to jump and be ridden. Last I heard he had finally been "retired" from the school and sold to a small child for her to ride...
Let your horse decide :)
PH Cody
