Around 6 to 8 months, most children begin to show distress when they are away from their primary caregivers. Children may become more “clutchy” before parents leave, which may be followed by crying and searching for parents or primary caregivers after they’re gone. This distress is called separation anxiety, and it is a normal reaction that tends to get stronger when babies are between 10 and 18 months old. After 18 months, separation anxiety generally becomes less intense for most children. As children grow closer to age 2, they understand more about leaving and returning and can anticipate reuniting with primary caregivers when they come home again.
Weinraub, M., & Lewis, M. (1977). The determinants of children’s responses to separation. Monographs of the Society for Research in Child Development, 42(4), Serial No. 172.