In medieval society, childhood did not exist. Around seven years of age, they took the child as a little adult in the community with a similar expectation for a job, marriage, and legal consequences. We can give the crown for originating ideas of development to Charles Darwin, in recognition of his work on the origins of ethology (the scientific study of the evolutionary basis of behavior) and "A biographical sketch of an infant."
It wasn't until the 20 century that developmental theories came forth. When conceptualizing cognitive development, we cannot ignore the work of Jean Piaget. Piaget suggested that when young infants experience an event, they process new information by balancing between assimilation and accommodation. Assimilation is taking in new information and fitting into previously understood mental schemas while accommodation is adapting and revising the previously planned mental schema according to the novel information. Piaget divided child development into four stages. The first stage, Sensorimotor (ages 0 to 2 years of age), is the time when children master two phenomena: causality and object permanence. Infants use their senses and motor abilities to manipulate their surroundings and learn about the environment. They understand a cause-and-effect relationship like shaking a rattle may produce sound and may repeat it or how crying can make the parent(s) rush to give them attention. Soon with frontal lobe maturation and memory development, infants can make mental schemas and can imagine what may happen without physically causing an effect and thus plan out actions better (emergence of thought). Object permanence emerges around six months of age. It is the concept that objects continue to exist even when they are not presently visible. Then comes the "pre-operational" stage (2 to 7 years), when a child can use mental representations, symbolic thought, and language. The infant learns to imitate and pretend to play. He is egocentric, i.e., unable to perceive that people can think differently than him, and everything (good or bad) somehow links to him. This stage is followed by the "concrete operational stage" (7 to 11 years) when the child uses logical operations when solving problems, including mastery of conservation and inductive reasoning. Formal operational stage (12 years and up), suggests an adolescent can use logical operations with the ability to use abstractions. He can understand theories and hypothesize and comprehend abstract ideas like love and justice.
Some concerns to keep in mind when understanding child cognitive development and Piaget's stages, is poor generalizability of stages. For example, conservation may overlap between pre-operational and concrete operational stage as the child masters it in one task and not in another. Similarly, our understanding now is that a child masters the "Theory of Mind" by 4 to 5 years of age, much earlier than when Piaget suggested that egocentrism resolves.
Stages of Cognitive development (problem-solving/Intelligence): The word intelligence derives from the Latin word "intelligere," which means to understand or perceive. Problem-solving and cognitive development progresses from the establishment of object permanence, causality, and symbolic thinking with concrete (hands-on learning) to abstract thinking and embedding of implicit (unconscious) to explicit memory development.
Newborn to 2 months: At birth, the optical focal length is approximately 10 inches. Infants seek stimuli actively, habituates to the familiar, and respond more actively when stimuli change. The initial responses are more reflexive, like sucking and grasping. He can fix and follow a slow horizontal arc and eventually will follow past the midline. He prefers a contrast, colors, and faces, understanding familiar from moderately novel stimuli. As he habituates to the caregiver's faces, he develops a preference. He will stare momentarily where an object has disappeared from (lack of object permanence). At this stage, he prefers high-pitched voices.
2 to 6 months: Infants engage in a purposeful sensory exploration of his body, staring at his hands and reaching and touching his body parts. Thus, building on to the concept of cause and effect and self-understanding. He appreciates sensation and changes outside of himself with less regularity. As he masters his motor abilities, something happens by chance, and then he repeats it. For example, touching a button may light up the toy, or crying can cause the appearance of the caregiver. He will anticipate routines at this age.
6 to 12 months: Object permanence emerges as the infant looks for objects. He will look for partially hidden objects first (6 months) and then completely hidden, for example, will uncover toys and engage in peek-a-boo (9 months). Separation anxiety and stranger anxiety emerge as he understands out of sight is not out of mind. As his motor abilities advance, he further explores using his senses by reaching, inspecting, holding, mouthing, and dropping objects. He can manipulate his environment, learning cause and effect by trial and error, like banging two blocks can produce a sound. Eventually, he builds a mental schema (as Piaget suggested) and learns to use objects functionally, for example, presses a button intentionally to open and reach inside a toy box.
12 to 18 months: Around this time, motor abilities make it easier for the child to walk and reach, grasp, and release. He can explore toys to make them work. Novel play skills emerge. He imitates gestures and sounds, and egocentric pretend play emerges. As object permanence and memory advance, he can find a toy after witnessing a series of displacements and tracks moving objects.
18 to 24 months: As memory and processing skills advance and frontal lobes mature, he can now imagine outcomes without so much physical manipulation, and new problem-solving strategies emerge without rehearsal. Thought emerges, and there is the ability to plan actions. Object permanence establishes completely, and he can search for an object by anticipating where it may be, without witnessing its displacement. At 18 months, symbolic play expands from self, and instead of pretending to eat himself, he may give the teddy bear a bottle and can imitate housework.
24 to 60 months (Preschool years): During this stage, magical and wishful thinking emerges; for example, the sun went home because it was tired. This ability may also give rise to apprehensions with fear of monsters, and having logical solutions may not be enough for reassurance. Perception will dominate logic and giving them an imaginary tool, like a monster spray, to help relieve that anxiety may be more helpful. Similarly, conservation and volume concept lacks, and what appears bigger or larger is more. For example, one cookie split into two may be equal to two cookies. At this stage, a child also has a poor concept of cause and may think he got sick because he misbehaved. He is egocentric in his approach and may look at situations from only his point of view, offering comfort from his stuffed toy to an upset loved one. At 36 months, he can understand simple time concepts, identifies shapes, compares two items (e.g., bigger), and counts to "3". Play becomes more comprehensive from simple scripts of feeding a baby doll to going to the park. At 48 months, he can count to four, identifies 4 colors and understands opposites. At 60 months, pre-literacy and numeracy skills further, and he can count to 10 accurately, recites "ABC's by rote, and recognizes a few letters. A child also develops hand preference at this age. During the ages of 4 to 5 years, play stories become yet more detailed and may include scenarios from imagination, including imaginary friends. Playing with some game rules and obedience to those rules also establishes during the pre-school years. Rules can be absolute.
Age 6 to 12 years: During early school years, scientific reasoning and understanding of physical laws of conservation, including weight and volume develop. A child can understand multiple points of view and can understand one perspective of a situation. They realize the rules of the game can change with mutual agreement. There is mastery of basic literacy skills of reading and numbers are mastered initially, and eventually, around third to fourth grade, emphasis shifts from learning to read to reading to learn, and from spellings to composition writing. All these stages need mastery of sustained attention and processing skills, receptive and expressive language, and memory development and recall. The limitation of this stage is an inability to comprehend abstract ideas and relying on logical answers.
Twelve and above (adolescence): During this age, teens can exercise logic in a systemic, scientific way. They can apply abstract thinking to solve algebraic problems and apply multiple logics simultaneously to reach a scientific solution. It is easier to use these concepts to schoolwork only earlier. Later in adolescence and adulthood, these can also apply to emotional and personal life problems. Magical thinking or following ideal guides decisions more than wisdom. Some may have more influence from religiosity/moral rules and absolute concepts of right and wrong. Questioning the prevalent code of conduct may cause anxiety or rebellion and eventually lead to the development of personal ethics. Side by side, social cognition, apart from self, also is developing and concepts of justice, patriarchy, politics, etc. establish. During late teens and early adulthood, thinking about the future, including ideas such as love, commitment, and career goals, become important.