A baby's digestive system isn't fully developed, and some infants have a hard time absorbing sorbitol. If these infants are given a juice with a lot of sorbitol, their metabolic rate increases, as does their level of physical activity. That's not always a good idea.
The juice with the most sorbitol is pear juice, so you might want to hold back on that for a while. White grape juice is probably the safest to start a baby on.