What is different about a fetus at this later stage to deserve a rational soul?
Aristotle believed in a tripartite soul, with 'vegetable' souls and animal souls, supporting only in humans, the intellective soul.
So you would have to think a baby had not begun to have an intellective soul until some stage of development. For Greeks pneuma, breath, was often regarded as spiritual. Children could have survived at several months premature. I would expect some kind of link there. But I don't think the explicit derails are there in their texts.
See this similar discussion for direct quotes of relevance: Did Aristotle believe in an immortal soul?
Historically speaking, most cultures held that a child becomes 'alive' at the quickening of the fetus — when the woman can begin to feel the fetus move within her — which occurs roughly halfway through pregnancy. Religions that posit a soul usually placed its entry at that time. This is (interestingly) why most abortion laws limit abortions to the first trimester; they are trying to respect religious sentiments.
The idea that the soul enters the body at conception is a recent phenomenon, developed within US Christian fundamentalist and evangelical churches to oppose permissive abortion policies that followed Roe v Wade. It has no real historical or biblical roots, but is a typically hyperbolic assertion made for political advantage. That's not trying to suggest that it's right or wrong, merely that it is motivated by something other than scientific or philosophical reasoning.
"Delayed hominization" is a particular case of the larger problem plurality of forms vs. the unity of substantial form:
Ariew, Descartes among the Scholastics (Leiden; Boston: Brill, 2011), pp. 83-84:
- St. Thomas Aquinas: Man is a unity of single form (the rational soul).5
- Duns Scotus: Man is a composite of a plurality of forms (rational, sensitive, and vegetative souls)6
- Aquinas Summa Theologica, I, quaest. 76, art. 3.
- Scotus Opera Omnia, Opus Oxoniense, IV, dist. 11, quaest. 3.
Ariew, Descartes and the Last Scholastics p. 21 fn. 28:
the Augustinian and Franciscan doctrine [was] the plurality of substantial forms. John Duns Scotus and William of Ockham held the thesis that man is a composite of forms (rational, sensitive, etc.), a thesis previously rejected by Thomas Aquinas, who argued that there is just one form or soul in man (the rational soul), which performs the functions that the other souls perform in lower beings.
St. Thomas Aquinas, Summa Theologica I q. 76 a. 3 co.:
an animal would not be absolutely one, in which there were several souls. For nothing is absolutely one except by one form, by which a thing has existence: because a thing has from the same source both existence and unity; and therefore things which are denominated by various forms are not absolutely one; as, for instance, "a white man." If, therefore, man were 'living' by one form, the vegetative soul, and 'animal' by another form, the sensitive soul, and "man" by another form, the intellectual soul, it would follow that man is not absolutely one.