Nefesh HaChayim, Reb Itzele's Long Annotation

And that the tzaddik came first, and the lamed later, and finally the mem:—lo—if it were the will of the Holy One (blessed be He) that the supernal roots should perform their activities according to the plans designated for them, in the body of the person from the day he’ was born, it would have been very simple for the person to merit to receive the brains of the tzellem (as previously described). However, the will of the Holy One (blessed be He) was that all activities of the supernal roots in the soul-Neffesh, soul-Ruakh, and soul-Neshama, are at first essentially hidden, and they are not immediately active from the day of birth until the child grows up. And what we perceive with our senses is that as the infant grows (in the vessel of the body), so too matures his intellect, speech and actions.