Final answer:
Depth perception and the associated fear of heights begin to develop around six months old, with progression throughout infancy and early childhood.
Step-by-step explanation:
The development of depth perception and a fear of heights in infants is an important cognitive milestone. Around six months old, infants begin to show the ability to discriminate depth. At this age, they can not only pick up objects but also transfer them from one hand to another, pull themselves into a sitting position, and their vision approximates adult vision in acuteness. However, the ability to perceive spatial relationships in 3-D space continues to develop in infancy as they become more mobile and improve throughout early childhood. By three years old, children's motor skills allow for activities that indirectly signify solidified depth perception, such as climbing stairs one foot per step and engaging in more complex play that requires an awareness of spatial relationships.