drafts seem to be the thing for Jousting. Strength seems to be the most important followed by Endurance Int doesn't seem to matter and i'm not sure if speed helps but all the winning horses have it capped
As for age they seem to be around 10-11
As for your horse i'm not sure if it would be good in jousting seeing how the horse that's kicking everyones butt has Amazing for Strength and all other stats are wise this is it
http://www.virtualhorseranch.com/viewho ... se=1365278
I don't do jousting yet though but i was thinking about taking it up and those are just things i've noticed by looking through completed events
Hope this helps any