You would discipline a young horse the same as you would any other horse. Age doesn't matter when you use herd dynamics. In fact, it should be a bit easier to become the leader for a young horse than it might be to become a leader for an older horse who might be set in his ways.
I always like to say "Ignore the sassy stuff". If you start teaching your young horse something and she gets confused or frustrated, the only way she knows of telling you about her difficulties is by rearing or bucking or other "sassy stuff". We don't want to discourage her from telling us. We should acknowledge that she isn't learning what we are trying to teach her, but we should not reprimand the "sassy stuff". We ignore those actions from her and redirect her to do the right thing. You need to teach her in baby steps and set up building blocks along the way. If we start teaching a horse to lounge, we can't expect him to w/t/c in both directions flawlessly the first time we introduce the task. We start with only walking in one direction. That is baby step building block one. Then we might walk in the other direction. Baby step building block two. Then we'll add the trotting and eventually cantering. If at any point during our teaching process the horse shows us some "sassy stuff", we go back to what the horse knows. Ignore the sassy stuff and redirect the horse's actions so that they can be successful in what we are asking from them.
Now, if I'm working with a horse that knows how to do what I'm asking of it and he shows me some sassy stuff, then I might get after him a little bit. I want my horses to know that it's work time when I'm working with them. They need to keep their focus on the job at hand. Usually a stern "Hey!" or a tug on the rope will get them re-focused on work. Sometimes, I do let them play if I can tell that they are a little extra spunky. My trail horse will have a devilish look in his eye while lounging and I'll say "Let'er rip" and he knows that's his cue to play. He runs & bucks & farts like a wild baboon! When I say "whoa", he slams on the brakes and stands to look at me. And then we are ready to get down to work.
Perfect Jin N Scotch - 2004 APHA Palomino Overo Gelding - Western Pleasure and Showmanship
Hanks Rainy Sky - 1998 Black and White Tobiano Paint Gelding - relaxing rides and blazing trails