Since becoming a father of two young daughters, and seeing their personalities grow (from birth) to their current ages (2, 6) I can say that their personalities were fairly developed at around six months. Sure, they couldn't talk, but their actions and their crying (all infants seem to have different kinds of cry's based on the emotion they are feeling) I can conclude that a lot of personality, and motivation come programmed in your genetics.
The way both of our girls' personalities have been since they were infants has been amplified by experiences and learning. Now that they can communicate better, and we understand them more, our guesses as to how they would "turn out" are fairly accurate. It's almost scary as to how much has not changed with their personalities since birth.