You asked: Is it true that the soul enters the body when a baby is born?

Thank you for writing with this question!
In The Urantia Book, we learn that the soul is born when a person receives a Thought Adjuster. Here on Urantia, a child is about 5 years-old when that happens. Here is that reference:
108:2.1 Though the Adjusters volunteer for service as soon as the personality forecasts have been relayed to Divinington, they are not actually assigned until the human subjects make their first moral personality decision. The first moral choice of the human child is automatically indicated in the seventh mind-adjutant and registers instantly, by way of the local universe Creative Spirit, over the universal mind-gravity circuit of the Conjoint Actor in the presence of the Master Spirit of superuniverse jurisdiction, who forthwith dispatches this intelligence to Divinington. Adjusters reach their human subjects on Urantia, on the average, just prior to the sixth birthday. In the present generation it is running five years, ten months, and four days; that is, on the 2,134th day of terrestrial life.
After the Adjuster is bestowed, then begins the work of growing the soul. I have bolded passages in the following references:
111:1.1 Though the work of Adjusters is spiritual in nature, they must, perforce, do all their work upon an intellectual foundation. Mind is the human soil from which the spirit Monitor must evolve the morontia soul with the co-operation of the indwelt personality.
111:2.4 There are three and not two factors in the evolutionary creation of such an immortal soul. These three antecedents of the morontia human soul are:
1. The human mind and all cosmic influences antecedent thereto and impinging thereon.
2. The divine spirit indwelling this human mind and all potentials inherent in such a fragment of absolute spirituality together with all associated spiritual influences and factors in human life.
3. The relationship between material mind and divine spirit, which connotes a value and carries a meaning not found in either of the contributing factors to such an association. The reality of this unique relationship is neither material nor spiritual but morontial. It is the soul.
And Jesus tells us more about the birth of the soul. I have bolded the part about the birth of the soul:
133:6.5 (1478.4) “The soul is the self-reflective, truth-discerning, and spirit-perceiving part of man which forever elevates the human being above the level of the animal world. Self-consciousness, in and of itself, is not the soul. Moral self-consciousness is true human self-realization and constitutes the foundation of the human soul, and the soul is that part of man which represents the potential survival value of human experience. Moral choice and spiritual attainment, the ability to know God and the urge to be like him, are the characteristics of the soul. The soul of man cannot exist apart from moral thinking and spiritual activity. A stagnant soul is a dying soul. But the soul of man is distinct from the divine spirit which dwells within the mind. The divine spirit arrives simultaneously with the first moral activity of the human mind, and that is the occasion of the birth of the soul.”
And so we see that the soul does not start to grow until a child is about 5 years-old and after he/she receives the Thought Adjuster. And it is the relationship between the Adjuster and the mind that gives birth to the soul and then grows the soul.
Thanks so much for this question.