Around 6 to 8 months, most children begin to show distress when they are away from their primary caregivers. This distress is called separation anxiety, and it is a normal reaction that tends to get stronger when babies are between 10 to 18 months old. There are different opinions about why separation anxiety occurs, but many believe that children’s attachments to caregivers are strengthening during this time, and seeing them leave feels threatening. After 18 months, separation anxiety generally becomes less intense.
Shelov, S. P. (Editor-in-Chief). (2004). Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.