Final answer:
By six months of age, infants should be able to fix on and follow a target. At four months, infants can follow objects with their eyes. These developmental milestones may vary from child to child.
Step-by-step explanation:
By the age of six months, infants are able to fix on and follow a target. Their vision has improved to be almost as acute as adult vision, and they start to notice colors and show the ability to discriminate depth. Infants at this age may also enjoy vocal play and make two-syllable sounds like "mama" or "dada." They may show anxiety towards strangers as well.
At four months, infants are able to follow objects with their eyes. They can roll from front to side, lift their head, sit up with support, and hold their head steady. They also start making vowel sounds and may laugh and squeal with delight.
It is important to note that these developmental milestones can vary from one child to another, so it's normal if a child achieves these skills earlier or later than the average time frames.