Birth - 5 months:
Reacts to loud sounds, turns head toward a sound source, watches your face when you speak, vocalizes pleasure and displeasure sounds (laughs, giggles, cries, or fusses), makes noise when talked to.
6 - 11 months:
Understands "no-no,” babbles (says "ba-ba-ba" or "ma-ma-ma"), tries to communicate by actions or gestures, tries to repeat your sounds.
12 - 17 months:
Attends to a book or toy for about two minutes, follows simple directions accompanied by gestures, answers simple questions nonverbally, points to objects, pictures, and family members, says 2-3 words to label a person or object (pronunciation may not be clear), tries to imitate simple words.
18 - 23 months:
Enjoys being read to, follows simple commands without gestures, points to simple body parts, understands simple verbs such as "eat," correctly pronounces most vowels and n, m, p, h, says 8 to 10 words (pronunciation may still be unclear), asks for common foods by name, makes animal sounds, starts to combine words such as "more milk," begins to use pronouns such as "mine."
2 - 3 years old:
Knows about 50 words & says about 40 words at 24 months, knows some spatial concepts such as "in," "on;" knows/uses pronouns such as "you," “I,” "me," "her;" knows descriptive words such as "big," "happy;" answers simple questions, speaks in 2-3 word phrases, begins to use plurals such as "shoes" and regular past tense verbs such as "jumped."
3 - 4 years old:
Groups objects such as food or clothes, identifies colors, uses most speech sounds, strangers understand much of what is said, describes the use of objects such as "fork," uses verbs that end in "ing," such as "walking," answers simple questions such as "What do you do when you are hungry?" and repeats sentences.
4 - 5 years old:
Says about 200 - 300 different words, describes how to do things such as painting a picture, defines words, lists items that belong in a category such as animals, answers "why" questions, Understands spatial concepts such as "behind," and "next to."
*adapted from the National Institutes of Health