Do you have kids? All three of mine had personalities within months, oldest one is a weenie, second is a dood and third is a frickin bulldog and those core personality has been true to this day. As for pain its often easily forgotten (you can remember the experience but not the actual pain) and is a learned response or its the interpertation of the pain...like "pain is weakness leaving the body"...early on with all I was...ehh its nothing...its far from your heart...ehh yer not going to die today. Oldest....a weenie that wines and rides but is not a rider, second ...come on mfker I can take it and third...it may hurt for a minute but I ain't letting go...and she is tougher than her two older brothers.
You seem to have behavior and personality confused. Behavior is a response due to an internal/external stimulus while personality is the perception of oneself and everything else. Personality requires depth, built from experiences acquired during life. Behavior, knowledge, cultural, language, etc. affect personality at all times so to say a infant of only a couple months has a personality is ludicrous. At age 3, a child is beginning to develop personality. This is accomplished due to trying different behaviors and attempting interaction/communication with peers and older individuals. Although behavior can be an early indication of personality, one can easily manipulate an outgoing child (between ages 1 to 4) to a complete introvert by reinforcing certain behaviors. Instead of basing your argument off of an insignificant sample (3 children), do some research on cognitive and psychosocial development.
Mike brings up an interesting point. I would assume (<-keyword) skiing would be less physically demanding compared to snowboarding at her age, but I bet what is "naturally" comfortable also plays a part. At this time, I'd have her try both and see which one she picks up the quickest and prefers. Once she hits age 5+ she should have enough muscle development to seriously take on either. I'm still jealous every time I see kids tearing it up on snowboards; I could have added 10 more years to what I have now