I would say it depends upon the horse. Some are done by 15 while others are still competing into their 20s. We have a mare at our barn that is 28 and while being kept only at the trot these days, still goes out on the trail almost daily. This same mare was still jumping lower fences, about 2'6, into her early 20s.
If you are not looking at competing, or only will do so on occasion, 12 is a good age. As was pointed out the horse has miles under his hooves and tends ot lack that young horse silliness..though even older horses are no guarantee in that line. And hey, don't cross ye old thoroughbreds off the list :)...they can be just as settled as ye old quarterhorses.
I went younger with my new horse, 6 at purchase a few months back turned 7 in early March, just becasue of what I wanted to do and an older horse at 10+ would have seriously cut into the long term goals. Fortunately he is a level headed thoroughbred who wasn't raced into oblivion.
Last edited by tlkng1; 03-31-2012 at 08:35 AM.