Final answer:
Infants realize they are separate from their mothers around six months of age, as part of the sensorimotor stage of development.
Step-by-step explanation:
Around six months of age, infants begin to realize they are separate from their mothers. This milestone is part of the sensorimotor stage of development, which lasts from birth to about 2 years old. At this age, infants also start to develop object permanence and may show anxiety towards strangers.