Cognitive development in early childhood can be described through observable milestones: how children respond to people, communicate, explore objects, and solve simple problems. The CDC defines a developmental milestone as something most children—at least 75%—can do by a certain age. The statistics below use that CDC definition and cover ages 2 months through 30 months.
Contents
- How to read these milestone statistics
- Early social and attention skills at 2 and 4 months
- Exploration and communication at 6 months
- Social understanding and object search at 9 months
- Language and problem-solving at 2 years
- The 30-month language benchmark
- Screening ages in the CDC guidance
How to read these milestone statistics
These are age-linked milestone observations, not a ranking system or a measure of intelligence. “Most” means 75% or more of children in the CDC milestone framework can perform the described behavior by the stated age. A milestone therefore gives families and professionals a practical reference point, while individual development can vary.
The age labels are important. A behavior listed at 2 months is not being compared with a behavior listed at 2 years. The skills become more complex over time: infants first attend to faces and voices, then use sounds and gestures socially, and later combine language with memory, symbolic play, and simple problem-solving.
The CDC source labels used for the age groups are CDC Milestones by 2 Months, CDC Milestones by 4 Months, CDC Milestones by 6 Months, CDC Milestones by 9 Months, CDC Milestones by 2 Years, and CDC Milestones by 30 Months. The supplied CDC facts do not include direct source URLs, so no links are added here.
Early social and attention skills at 2 and 4 months
At 2 months, the CDC describes a cluster of early social, sensory, communication, and movement milestones. Most babies calm down when spoken to or picked up, look at a caregiver’s face, seem happy when a familiar person walks up to them, and smile when someone talks or smiles at them. These behaviors place social attention at the center of the earliest milestone group.
Most babies by 2 months also make sounds other than crying and react to loud sounds. In visual attention, most watch an adult as the adult moves and look at a toy for several seconds. Physical milestones in the same age group include holding the head up while on the tummy, moving both arms and both legs, and briefly opening the hands. Together, these 11 observations show how early development spans several domains rather than language alone.
By 4 months, the social signals become more intentional. Most babies smile on their own to get attention, chuckle when someone tries to make them laugh, and look at a person, move, or make sounds to get or keep that person’s attention. The CDC also lists reciprocal vocal behavior: most make cooing sounds such as “oooo” and “aahh,” make sounds back when spoken to, and turn their head toward the sound of a voice.
The 4-month group includes feeding and object-related behaviors as well. Most babies open their mouth when hungry and they see a breast or bottle, look at their hands with interest, hold a toy placed in their hand, bring their hands to their mouth, and push up onto their elbows or forearms while on their tummy.
| Age | Social or communication examples | Exploration or movement examples |
|---|---|---|
| 2 months | Smiles in response to talking or smiling; makes sounds other than crying | Watches movement; looks at a toy for several seconds; holds head up on tummy |
| 4 months | Smiles to get attention; coos; makes sounds back when spoken to | Holds a toy placed in the hand; brings hands to mouth; pushes up on forearms |
Exploration and communication at 6 months
The CDC’s 6-month milestones show more active social recognition and exploration. Most babies know familiar people, like looking at themselves in a mirror, laugh, take turns making sounds with another person, blow raspberries, and make squealing noises. The combination of recognizing people and participating in vocal turn-taking marks a more interactive stage of early communication.
Object exploration is also prominent. Most babies put things in their mouth to explore them and reach to grab a toy they want. These behaviors connect sensory investigation with purposeful movement: the child is not only reacting to an object but also moving toward it and examining it.
The 6-month list includes a clear early communication signal around feeding. Most babies close their lips to show they do not want more food. This is a nonverbal way of communicating a preference. The CDC also reports three body-movement milestones for most babies by 6 months: rolling from tummy to back, pushing up with straight arms while on the tummy, and leaning on the hands to support themselves while sitting.
These 12 milestones illustrate why cognitive development statistics are best read alongside social, communication, sensory, and motor observations. Reaching for a desired toy, taking turns with sounds, and using the mouth to explore are different behaviors, but each provides information about how an infant is engaging with people and the surrounding environment.
Social understanding and object search at 9 months
By 9 months, most babies show a broader range of emotional and social responses. The CDC reports that most are shy, clingy, or fearful around strangers; show several facial expressions such as happy, sad, angry, and surprised; look when their name is called; and react when a caregiver leaves by looking, reaching, or crying.
Play and communication become more varied at this age. Most babies smile or laugh during peek-a-boo, make many different sounds such as “mamamama” and “bababababa,” and lift their arms to be picked up. These facts describe social anticipation, emotional expression, vocal experimentation, and a gesture that communicates a clear request.
The 9-month cognitive and exploratory observations include looking for an object after it drops out of sight and banging two things together. Searching for a hidden object is an early example of continuing attention when the object is no longer visible. Banging objects together is an active way to explore what happens when objects are combined.
Most babies by 9 months also get into a sitting position by themselves, move things from one hand to the other, and use their fingers to rake food toward themselves. These three movement and hand-use milestones accompany the social and object-focused behaviors, showing that attention and exploration depend on growing physical control as well.
Language and problem-solving at 2 years
The CDC’s 2-year milestones move from infant vocal and gesture patterns toward language linked to meaning. Most children by 2 years point to things in a book when asked where the bear is. This milestone involves understanding a spoken question, locating a named or pictured subject, and indicating the answer.
Most children by 2 years say at least two words together, such as “More milk,” and point to at least two body parts when asked. The first fact describes combining words to communicate a request or idea. The second links receptive language—the ability to understand a request—with body knowledge and pointing.
Gestures also become more varied. Most children by 2 years use more gestures than waving and pointing, including blowing a kiss or nodding yes. These behaviors show that communication at this age includes speech and meaningful nonverbal signals.
The CDC also lists several practical problem-solving and play behaviors. Most children hold something in one hand while using the other hand, such as holding a container and taking off its lid. Most try to use switches, knobs, or buttons on a toy. Most play with more than one toy at the same time, such as putting toy food on a plate.
| 2-year milestone area | CDC examples |
|---|---|
| Understanding and pointing | Points to a picture in a book when asked; points to at least two body parts |
| Spoken language | Says at least two words together |
| Gestures | Uses gestures such as blowing a kiss or nodding yes |
| Hand coordination | Holds an item while using the other hand to remove a lid |
| Cause and effect | Tries switches, knobs, or buttons on a toy |
| Multi-object play | Uses more than one toy at the same time |
The 30-month language benchmark
The supplied CDC milestone list includes one quantified language benchmark for 30 months: most children say about 50 words. The phrase “about 50” is an approximate count, not a statement that every child has exactly 50 words. As with the other milestones in this article, “most” refers to 75% or more under the CDC definition.
This benchmark follows the progression visible in the earlier age groups. At 2 months, most babies make sounds other than crying. At 4 months, most make cooing sounds and respond vocally when spoken to. At 9 months, most produce many different repeated sounds. By 2 years, most combine at least two words. At 30 months, the supplied CDC statistic gives an approximate vocabulary size of 50 words.
Because the age and measurement wording change across these milestones, the figures should not be treated as a mathematical growth curve. They are separate CDC reference points describing behaviors that most children can perform by each age.
Screening ages in the CDC guidance
The CDC recommends general developmental screening at 9 months, 18 months, and 30 months. It also recommends autism screening at 18 months and 24 months. These are screening time points in the CDC developmental guidance, distinct from the milestone observations themselves.
| Screening type | CDC-recommended ages in the supplied guidance |
|---|---|
| General developmental screening | 9 months, 18 months, and 30 months |
| Autism screening | 18 months and 24 months |
The overlap at 18 months matters because the supplied guidance identifies that age for both general developmental screening and autism screening. The milestone lists in this article provide age-based observations through 30 months, while screening recommendations identify points when structured developmental review is recommended. They serve related but different purposes: milestones describe commonly observed abilities, and screening is a formal check conducted at specified ages.