My two year old is the sweetest thing in the world. She has the temperament of a 22 year old, learns EXTREMELY fast, etc.
Every time my trainer comes out to do lessons he always reminds me how well mannered she is.
Faith and I were in the arena one day I was just sitting on her, and I was just thinking 'Gee I wonder what she would do I if I just squeezed with my left leg and leaned my left rein against her neck....' and she started to do a pivot! Both sides! She's very smart and I'm very lucky to have a horse like that, I know many other young horses who are just nuts.
Just depends on the individual horse, etc. It also depends on the person who handles it, if your going to be hyper then the horse will be hyper, if your going to be calm then that will wear on the horse.
Remember its like nuts and bolts. If the rider is nuts, then the horse bolts!
Also not trying to pick up the past or anything, but didn't you have a thread about a yearling that would rear, etc.? If so (and if this that same horse) I suggest getting a trainer soon, it sounds like the horse might need it.