On to what, if one could be said to have favorites in this context, the theorist who would have to be mine, Jean Piaget, and our cognitive development. The first stage of cognitive development is called the sensorimotor stage, and is characterized by the engagement in the physical world. This stage is concerned with the development of two primary milestones: object permanence and language acquisition. Object permanence is the ability to conceive that an object, or person, continues to exist even when it is out of sight. This initial feat of memory is the beginning of our mental gymnastics across time. The second, language acquisition, is not strictly speaking the full access of language, so much as the beginning to express themselves in symbols. This is the very first step in abstract thought, the ability to turn a concept into a symbol.
This moves us into the pre-operational stage, where language, memory and imagination all expand. Language grows as kids get better at representing concepts with symbols, and the combination of symbols. Memory and imagination are the representation of moving forward and backward in the mental time space. They can distinguish past from future, but still struggle with before and after. These mental skills can be intentionally fostered with creative tasks, trying to find new styles of symbols to represent concepts; exposure to lots of words and numbers; and memory and story telling games. This is another encouragement at this range recommending the creative expression, particularly story telling.
The concrete operational stage introduces a further increase in abstraction ability, which supports the concept of time, cause and effect, and comparison. This allows the idea of disagreement, and that other people's thoughts might not be the same as their own, which should also be reinforced. This age corresponds with Erikson's fourth stage, so this skill of recognizing that other people can have different opinions is a skill that can be praised and established as a core life skill. The abstraction skill is still not fully developed, and at this age children still struggle in situations with multiple variables.
Piaget's last stage of cognitive development is the formal operational stage, also around the onset of puberty. This stage marks a considerable upgrade in the abstraction capacity, allowing for complex hypotheses, and future planning, conceptual ideas like loyalty and justice, metaphor and analogies. Piaget was emphatic in his claim that this was the end of cognitive development, and all future development involved increases in knowledge. While I have been trying to stay rooted in the historical theories it is important to update the fact that we know that neural development, your physical brain hardware, has another significant upgrade waiting around age 25, which offers the last installment of both abstract thought and, significantly for what we are about to discuss, self-regulation. It is important that we all know that you do not have a full brain until you are around 25, and that impulse control, autonomous decision making, and synthesizing of information are all governed by the part of your brain that is the last to develop. That is the last of Piaget's historical model of cognitive development, but more cognition related stuff will come up while we discuss behavioral development.