We have trained nothing but trail horses since we got too old and are in too poor health to train cow horses and reining horses any more. We always rode our cow horses out and they were perfect trail horses and we sold the horses that would not make competitive cow horses as trail horses for many years - about 35 or 49 years anyway. Now, that is all we can do.
It does not take age. We have had MANY 2 year olds that would go anywhere you pointed their heads. I have sold 3 year olds to novice riders that are still perfect trail horses 10 years later. [I got 2 e-mails just last week from people that bought horses 3-5 years ago and keep me up on their adventures. Both of those horses were 3 year olds. ]
'Almost' any horse will make a good trail horse. Some super paranoid, exceptionally spooky horses will always need a confident rider, but I have not had a problem making a good trail horse out of anything. I have made good trail horses out of many spoiled horses, but that takes a lot more skill and riding ability than what many people have. Obviously, the nicer the prospect and the better the attitude, the easier it is to make a nice horse for any purpose. We raise our own prospects for their trainability, good minds and easy going nature. We think novice riders should have that kind of horse because they are 'user friendly' and 'low maintenance'. Those are inherited characteristics.
Horses with 'big motors' like TBs and race-bred QHs and high strung horses also require more rider skill, but they cover a lot of ground and are really more suitable to those wanting to do endurance and long hard rides. If you wanted a vehicle to go fishing and hunting in and drive into the back-country, you would not buy a Corvette or a Ferrari would you? Those 'hot' horses make really fast mounted shooting horses and the ones with speed make barrel horses and other timed event horses. They just require a rider with greater skill.
Here are the best tips and 'rules' I have for making a good trail horse:
1) Obedience is NEVER optional. A good trail horse is nothing more than a horse that does everything 'right away' that a rider asks. Absolute and quick obedience -- 100% compliance without an argument should be the goal.
2) Your job (as the rider) is not to let your horse look at everything new and decide it is OK. That is your job. You should NOT show him that there is nothing to be afraid of. Your job as an 'effective' rider is to teach him that he needs to trust YOU and ONLY YOU -- not his natural instincts. It is your job to teach him to pay attention to his job (doing whatever you ask) and not his surroundings. Your goal should be to teach him to ignore anything he 'perceives' as fearful.
3) I NEVER let a horse look at things, examine things, go up to new things, 'sniff'' things or any of that. If you do any of these, you are teaching to stop and look or sniff everything instead of go on down the trail. The habit I want to reinforce is to go past or through anything without stopping to look at it. If I tell him it is OK, I want him to accept that without questioning me. You can't have it both ways. He either has to become the leader and figure out everything for himself in his time-frame (for some horses that is never) or he has to let you be the leader. I am convinced that I am smarter and know what I am doing and I know where I want to go and I don't really need or want his opinion at all.
If you let a horse look at things, then you are teaching him to be afraid of everything that is new and telling him that things should be looked at instead of ignored. You are not telling him that it is OK to go right past it. I want a horse to ignore everything but me. You have to remember that whatever you let or ask him to do (like checking things out) is what you are teaching him to do. Do you want a horse that is afraid of everything and stops at every new thing he encounters or do you want a horse that goes everywhere you point his head without questioning you? Remember, you just can't have it both ways.
4) When a horse starts to hesitate and starts to show fear, 'ride hard and fast'. Go faster, cover more ground, ride off of the trail and in the roughest footing you can find. All of these things get his attention back to his 'job' and back to you and off of whatever he thought was a big wooly booger.
5) Never ride straight toward something that you can go around. If a horse is afraid of a big tree stump, do not ride him straight toward it. [You are just setting his up to stop and back up. Remember, you are trying to make the right thing easy and the wrong thing difficult and setting him up to stop and back up is not doing that.] Ride past it several times while taking his attention away from the stump and keeping it on you. I like to use 'leg yielding' exercises. I will ride past an object with his head bent away from the object and my leg pushing his shoulders and ribs toward the object. I watch his ear that is away from the object. I know I have his attention and respect for my leg when that ear stays 'cocked' back toward me. I will go past the object, switch my dominant rein to the one nearest the object, will reverse directions TOWARD the object (I never let him turn his tail to anything he fears) and I will leg yield back past it again using my other leg to push him (bend him) toward it. I will go back and forth again and again until he walks right on by without looking at it or veering away from it -- just goes straight on by like it isn't there.
We help a lot of riders get past their fears on the trail. When you have an apprehensive rider that is possibly more fearful than the horse, you cannot expect that person to project a confident 'git-er-done' bold demeanor to the horse. So, the rider has to learn how to ride past their fears, focus on a place way past where they are and ride with determination to that place. You want to concentrate on getting to a place that is far beyond the object that the horse is trying to focus on. If the rider is looking at a 'booger', you can bet that the horse is going to be looking at it, too. Many people 'spook' worse than their horse. They are looking for scary objects down the trail before their horse is. If that is part of a rider's problem, they need to learn to ride far ahead of where they actually are.
We do not spend a lot of time trying to desensitize a horse. A lot of people find this strange. Let me tell you why we put so little faith in this exercise in futility (and why I never post on those threads). You will never be able to duplicate everything that can scare a horse. Even if you did, they would encounter this obstacle in a different place on the trail and it would be different to them anyway. You train a horse to listen to you and you train a horse to ignore anything new or scary. You train a horse to go forward when you ask -- no matter what is in front of them (one of the reasons I keep harping on 'good forward impulsion' ) and you train a horse to depend solely on you. You make all of the decisions and they are happy to comply. The more you take the leadership role, the less they think and worry. That is how you make a good trail horse.