Final answer:
Infants typically achieve the milestones of saying their first word, sitting without support, and teething between the ages of 6 to 12 months, with first words often being spoken around the end of the first year.
Step-by-step explanation:
Developmental milestones in infants are marked by significant growth and achievements. Typically, by the age of six months, infants engage in behaviors such as babbling, sitting, and noticing colors and depth changes. They also show the beginnings of teeth development.
As they approach their first birthday, around 12 months, infants often reach important milestones which include standing with assistance, potentially walking, and saying their first words. This verbal expression frequently starts as simple but meaningful utterances like "mama" or "dada." Also, infants demonstrate cognitive abilities such as following simple commands and showing a recognition of familiar objects or events. The span of time between six to twelve months is filled with rapid learning and growth, as infants start to interact more meaningfully with their environment and caretakers.