Aren’t baby bones mostly cartillage? IIRC, the Spartans would decide on its fate immediately after the birth of the baby.
No, you must be thinking of a fetus. The embryo starts out with a skeleton of primarily hyaline cartilage, but by the time of birth most of this hyaline cartilage has been converted to bone with the exception of ephiphyseal plates near the end of long bones such as in the limbs. This allows the bone to grow. During adolescence, this too converts to bone, and the bone stops growing.
At least the infant skull is bone. The professor showed us one in a human skeletal biology class. So tiny. And it made me feel like crying, because some woman had lost that baby once.