Going back to Arabians and endurance, in 2002-ish there was a 28 year old Arab/QH cross that was a close runner up in one of those big endurance races and it came in with really good condition and stuff, so it can be done with an older horse.
Just thought I'd get that out there.
I personally ride an Arabian and she has spunky moments, like any horse, but she is also one of the smartest horses I've ever ridden. She doesn't spook at all if she can see the scary thing well, and she picks up on all the exercises I do with her lightning fast. If you want a really smart horse I'd say look into an Arabian, but they aren't for everyone.