It is generally inferred that in any individual infant the Babinski sign may be present at birth and may then remain demonstrable for an unpredictable time until it permanently disappears, before the age of 2 or 3 years.
It is usually agreed that this early present sign is due to lack, or irregularity, of myelinization of nerve fibers and that the disappearance of the sign marks the maturation of myelinization.
It has been repeatedly stated that the occurrence of a spontaneous Babinski sign in the young infant is pathological.
DIETRICH HF. A Longitudinal Study of the Babinski and Plantar Grasp Reflexes in Infancy. AMA Am J Dis Child. 1957;94(3):265–271. doi:10.1001/archpedi.1957.04030040051007