True enough perhaps for an average person, but that isn't including personal tragedy. I seventeen year old who watches his/her family get murdered is not going to be the same after that. Even in general there's a huge difference between a child, a teenager, a middle aged adult and an elderly adult. A man at sixty is not the same man he was at thirty nor at fifteen.
In ST:V when offered the chance to have all his past mental anguish taken away by Sybok, Kirk said something to the effect of "I need my pain, it makes me who I am." Without witnessing the massacre mentioned in "The Conscience of the King", choosing duty over friendship as mentioned in "Court Martial", being forced to let Edith Keeler die to fix the timeline and all the things unmentioned Jim Kirk would not be Jim Kirk. Even the way he talks, his body language and personal "ticks" were probably adopted during his career.
Some "core" elements of Kirk's personality are his willingness to cheat to win, break the rules for the greater good, sacrifice himself for others, never say die attitude, drive to be the best and let's not forget the lady killer aspect. A teenage Kirk was likely a smooth talking, competitive, cocky kid with an eye for the girls and a penchant for getting himself in trouble. No need to even meet Shatner to pull that off, just grab the guy who played Ryan on the OC tell him to drop the brooding routine and slap a uniform on him, casting problem solved.
Just my opinion mind you, I'm probably way off. Wasn't Kirk described in one episode as being a "bookworm/stick in the mud" at the academy? What changes a bookworm into a galaxy saving, brawling, dashing, ladies-man of a starship captain?