@AvrohomYitzchok asked me this question to support the assertions for another question: "Why don't newborns speak? Why do we need to learn a language?"
In my understanding, in Judaism, a soul is a complete entity having only one degree of freedom, namely the total amount of merits and sins. The soul is unchangeable otherwise in its other qualities. It dwells happily in the Treasure of souls until God decides to bestow it a chance of ascending to a better spot by coming down to this world and participating in the game of human history, getting rewards for observing Mitzvos and getting fouls for overriding it like in a video game, but the character itself stays unchanged through the game.
I also understood from my Rabbi's Kabbalistic teachings, that studying Torah and keeping Miztvos in this world does not change the soul, but only affects its merits and its sins do not affect the soul but the cleansing it gets. This also explains the reincarnation of the souls of seniors to kids.
For some comments to my question, I learned that some speculate that the soul might grow just like the body does, and the soul that comes down is a kind of a "baby-soul" that develops further into a "full-grown" soul.
What do our sources say about the soul's development over a lifetime?