Final answer:
Infants typically begin to show stranger anxiety at around six months of age. This period coincides with cognitive and physical developmental milestones that also emerge around the same time.
Step-by-step explanation:
Infants begin showing stranger anxiety at about six months. Stranger anxiety is a developmental milestone where infants start to exhibit fear or distress in the presence of unfamiliar people. This behavior aligns with cognitive milestones, such as the development of object permanence, which also occurs around this age. At six months, infants have improved vision, can engage in vocal play, and demonstrate physical skills like transferring objects from hand to hand. They start to recognize familiar faces and may show anxiety toward strangers by crying, clinging, or reaching for a caregiver.