Balancing Development and Preventing Explosion

I had someone reach out to me about asking me to elaborate on balancing the horses individual ability and mental development. Specifically, the concept surrounding the horses' physical abilities being further ahead of what their temperament or mental status is ready for.  I do believe this could stand for any breed of horse, some physically are bred to develop quicker than others. I've seen Quarter horses at 3 years old be more mentally together than my 10 year old FEI horses. I think this important to consider when purchasing a prospect-- do the research and find a prospect that will give you the temperament that you seek to do what you want to. 
I know there is a huge amount of debate on when you should start horses, and warmbloods in particular are slow maturing horses not just physically but also mentally. Ideally, I like to sit on them (and maybe walk) in the roundpen by the end of their 2 year old year. Keep in mind-- we are not working-- they are not going 5 days a week for an hour. As 3-year-olds we spend the WHOLE YEAR working on W/T/C and getting them out to the arena to learn steering. I will say in the many horse I have started that I have rarely had fireworks(bucking, rearing, etc). Generally, at this point if there are fireworks you are generally overwhelming them, OR the weather/ etc is not suitable for their brain to be on your side. Read them on the ground before you just swing a leg over. 
I will say generally, between their 4 year old and 6 year old (sometimes even 7), there will be "turbulence". I generally refer to this time as their "teenage dirtbag" years. Generally, they have gained a decent bit of confidence and you are at the same time starting to teach them "big kid" dressage. Sometimes when you ask the horse to do something, you will get "push back". When I start teaching new things that may cause their mental battery to become depleted and then potentially cause some turbulence, I try to maintain a system to my training. It is not that I don't address the topic-- whatever is causing them to feel overwhelmed. The most common turbulent training movement is the flying change-- lots of horses become very fried and upset when learning these. It is so very important that we do push the horse to try new things that may tax them mentally, but always come back to something they know so they can keep their confidence. So, in the case of the flying change, when I start them I only school them maybe 1/2 times a week. I check my counter canter and overall canter rideability (something they KNOW) before asking, after asking, and on the days we don't school the changes. Why? Because while the horses are allowed to become frustrated with a new idea/movement/etc, they must become gradually willing to accommodate new ideas and pressures and know that they can come back to something they know. 
When you mentioned that the horse has physical ability, but not mentally able to keep up-- how do we manage? We begin testing and asking the mental state if we can broaden what it feels comfortable with without diminishing the horse's overall confidence. It is like anything else-- If I go to the gym and only ever lift the weight I feel comfortable with, then how much more weight am I really going to be able to push -- how much muscle will I build? The brain is the same way-- I HIGHLY recommend reading The Talent Code  to go into more depth about the brain and developing skill. 
I do not develop my horses with ANY consideration on young horses, developing horse classes. That is a personal choice, but I have found that a 6 year old doing 3rd tends to have a lots of holes in training and is generally not confident at the level. Who wants to show a horse who isnt confident in its work? Does this help them progress? In my opinion(and its my blog), I would say no. The training scale is an important thing we need to keep in mind when developing, for sure. When I say I develop at their own pace, I do not mean that I never "push" them. It means more so that I dont come into every ride with an expectation that we are going to necessarily work on X, Y, or Z. If we train a movement which is difficult for the horse that day, then it is only because the warm-up and other things I rode felt good enough to proceed with more difficult maunevers. This is dressage-- there is a point to the levels. It is like school, everything builds on the things beneath it and the horses gain confidence within the principles and movements so that over time the more difficult work will not cause great distress to the horse or over tax them physically. I have plenty of days where my FEI horses come out and I feel they are not adequately supple, or they are pulling with their shoulders, or when I ask for a shoulder-in they want to duck behind my leg. That means on that day I will not proceed in my work until that issue is resolved and I feel like the horse is understanding of what is wanted again. If I get a good feeling in the warm-up(which becomes more extensive as they become higher level) then we will proceed with more difficult training. 
What should you do if there is "turbulence"? Depends on the Turbulence:
1. If the horse is not adequately listening to the aids, the aids have to become more clear(sharper at times, but NEVER abusive or excessive). If the horse reacts negatively to this correction, we allow them to have their "moment" and do not address it, move past it. It is important to not punish a horse's negative reaction to a correction as it will escalate the issue and diminish confidence. 
2. If the horse is overreacting or behaving in a way that is out of the norm- check equipment and other things that could cause discomfort before reacting to the horse's behavior. 
3. Sometimes, we have to push the horse through the turbulence. Say I put my leg on a horse and they initially jigg or try to "run" away from it, then I will keep that leg next to the horse til it relaxes. If the horse starts to become upset, I will use vocal reassurance, relax my seat, and scratch the neck but won't remove the leg until the horse relaxes. 
​So, in short to answer the question posed, a horse that is physically able but mentally behind should be gradually challenged to expand their mental willingness in a clear and kind manner. We want to always bring clarity to them and so the most important part is to always be clear in what you are asking the horse and the response you hope to achieve. Then from there you REWARD THE EFFORT NOT JUST THE EXPECTATION. Meaning, if the horse gives a half right response, a step in the right direction but not totally there yet, we remove the aid(pressure) and then reward. This is what is going to encourage the horse to try again, even if they become frustrated. Eventually they will get it right, over time they will become more at ease with the new idea, and in some cases, it will even become the thing which you use to help settle them when learning new things in the future. 

---