Does the human soul come preformed in infants or does it develop over a person's lifetime?
Of course you know that the existence of the soul is not even scientifically logical nor proven. Everything written about it is therefore only in philosophical thought. Different cultures have different beliefs, yet most agree that yes, indeed , soul exists. Many believe that souls are reincarnations, so man of us have old souls or get some baby souls every now and then. Some believe that they can see the wisdom of the souls in their relatives already even when they are children so they can predict how these kids will fare in life. I personally feel that souls come preformed as you mentioned, but have absolutely no proof that this is even logical or possible.
If soul is the essence of GOD within man, infinite energy and life source, then it is fully mature and cannot be diminished. The term Soul as a substitute for Awareness develops within a lifetime.
Aristotle said the soul is the function of the form. Based on this, the soul would come preformed in infants.
Accepting a religious faith only accepts the soul in a human body as immortal. The souls as we are given to understand are preformed on entry to life on Earth and it does not have any development, except to leave a body to enter another one! So confusing and unscientific the explanations are, one does not understand what the soul means, really!
The Real Soul is beyond form... so in that sense the soul is preformed. Differences in personality come from the impurities that hide the pure immortal soul, and of course those things change over a person's lifetime. If you want to have a deeper understanding of soul, (the I AM) look at the latter chapters of this free online source...