I suppose it depends a lot on what the older horse has done.
An older horse, been there, done that, seen it all, that has also jumped and done obstacle- that would be a doddle to harness train.
When I was doing the big(ger) horses I preferred an animal that was saddle trained and had worked, it meant they were bridle wise, etc- mind you I have limited experience training big horses that someone else had done all that work on- I was invariably doing one I had bred and raised, so that would make a difference.
But a lot of older horses are better at a lot of things than youngsters, anyway.