We bought my mare unbroken at 14, so she had already finished growing, and she was amazingly tractable, just saddled her up and rode her off. Of course, she had no idea what any signals meant, and had to find her new balance, and all that. But she's got very smooth paces, a very soft hunting trot that's no trouble to sit, and a rocking chair canter. So it's easy to look good . . . or at least as good as I can.
I've about decided (after many years participating and watching this process) that most horses are fairly easy to break and train if you handle them right. Most of them are confused of course to begin with, but enjoy the learning if they are pushed hard.