Final answer:
Parents should notify their doctor if their infant isn't making vowel sounds like "ee", "ooo", and "ahh" by four months of age, as this may be a sign of a developmental delay in language.
Step-by-step explanation:
It is important to monitor an infant's development, including their ability to make sounds. By the end of their fourth month, infants typically start making vowel sounds like "ee", "ooo", and "ahh". Parents or caregivers should notify their doctor if the infant isn't making these sounds by four months of age, as it may indicate a delay in language development. It's also essential to note that by six months, infants usually start babbling, and by the first year, many are starting to say words such as "mama" and "dada".