Described by Sandra Dodd, for the assistance of unschoolers
The ages are general. Some children take longer at one stage than others, the same way some get taller sooner, some walk earlier, etc.
"Sensorimotor" stage: birth to two or three (some children longer)
"Preoperational" stage: language use to age 7, more or less.
"Concrete Operations": age 7 to puberty, give or take.
"Formal Operations": puberty into adulthood, but this isn't something everyone comes to. For one reason or another, some people don't develop logically as well or as early as others do. Don't worry about it, though. There won't be a test. And if Howard Gardner is right, then Piaget's set of developmental stages might only work ideally for those with a high logical intelligence.
Parents can make a big difference by helping children work through their thoughts and theories without scoffing or criticizing. Awareness of this pattern of development can help parents avoid expecting young children to think in ways of which they are incapable, and avoid holding children responsible for "understanding" or "agreeing to" things they can't really comprehend.
Some parents will say, "I explained it and he said he understood." What probably happened was the child heard "blah blah blah blah, okay?" and said "Okay."
In a discussion about helping two-year-olds realize and understand this or that about toys being there later, or about sharing and ownership, Pam Sorooshian wrote:
This is also a brain development stage, though. You cannot persuade a kid that things have "permanence" until their brain has developed the ability to grasp the concept. There are some fun little Piagetan experiments you can do with little children that help parents understand that you cannot force a child to understand something that their brain isn't capable of understanding.Joyce's responses to these statements in that same discussion:
My daughter has now a strong sense of ownership, she is saying more the words “it is mine” or “it’s my turn”. Maybe this is a reflection of me telling her “this toy is not yours” when she grabs the toy from the other child.
More likely it's the next stage of brain development. For some kids the stages—and their new behaviors—come gradually. For some kids it's like a light switches on and suddenly they see the world in a brand new way.
If you can get hold of the books Pam mentioned from the Gessell Institute by Louise Bates Ames, they describe what children are going through in each age well. It's a series title Your One Year Old, Your Two Year Old, Your Three Year Old, etc.
If you're not in America they might be harder to get, though!
[as to whether toddlers need to own things] Not own as in permanently belong to her. Own as in it's hers for now. If she's playing with a toy, she'll be upset if it's snatched away from her. If she's sitting in a chair she'll be upset if someone pushes her out and sits down.
While kids aren't miniature adults, they aren't alien creatures either. She understands the world differently than adults, but her reactions make sense from her understanding. And if you could understand how she sees the world, her reactions would make perfect sense to you too.
She isn't old enough to understand the toy isn't hers. She isn't old enough to understand she'll hurt someone if she seizes possession of something someone else is playing with. From her current understanding of the world it belongs to her and she'll react the same way an adult would if someone took something that belongs to her. That's why distracting them with something else works better at that age than reasoning with them :-)