In order to understand how children move between stages, it's important to understand how children take in stimuli from the environment and use it to grow. Most theorists agree that there are periods in children's lives in which they become biologically mature enough to gain certain skills that they could not have easily picked up prior to that maturation. For example, research has shown that babies and toddlers' brains are more flexible with regard to learning to understand and use language than are older children's brains.
Children are ready and open to develop certain things during specific stages; however, it doesn't just happen. Instead, they need proper environmental stimuli to develop these abilities. For example, babies have the ability to grow in length and weight in amazing amounts during the first year, but if they're not fed and nurtured enough during that time, they will not have the tools and building blocks to grow and will not grow and thrive. This is why it's so important for parents and caregivers to understand how their children are growing in all ways and channels and to know what stimuli, or stuff, they need to give their children to help them thrive.
From time to time children without any cognitive or physical problems at birth may not be able to develop certain milestones during the stage or time period they are most receptive. There may be an injury, illness, caregiver neglect or abuse, or a shortage of needs such as food or medical care, that make it difficult for a child to absorb all the basic building blocks and stimulation they need to gain certain abilities at certain times in life. When this occurs, affected children will generally have a harder time gaining those abilities even if they later get special attention and resources designed to help them compensate. It's like children have a window of opportunity when they are ready to grow in certain ways if they have the right stuff and tools in their environment. When that window closes, it will never be as easy to grow in those ways again. Theorists disagree about how important it is for children to have that special stimuli at each growing stage in order to reach their milestones. Some theorists call these times critical periods, but other theorists call them sensitive periods.
The difference between critical periods and sensitive periods is subtle. Theorists who believe in critical periods believe that children who do not get special stimulation during their window of receptivity are going to be "stuck" forever and never gain the abilities they should have gained in that period. However, other theorists believe that those very sensitive times in a child's life are just sensitive periods. They agree that children who do not get the right nurturing at the right times to jumpstart their developmental potential are going to have problems later in life, but they do not think that this inability to develop is permanent.
For example, infancy is the time when children first learn they can trust an adult or parent to take care of all their needs, keep them safe, and give them love. Some infants live in orphanages where there are far too many babies for the few nurses and staff members to take care of them. These children go through their first years with hardly any touch or affection that would teach them to trust and to show affection to caregivers. If these children are eventually adopted by a loving family later on in their childhood, they often have trouble adjusting to having an affectionate, loving parent. There have been many cases in which children who start out in that kind of orphanage environment never gain the ability to show affection and emotion toward family or even the ability to show remorse or compassion toward other people, no matter how loving and nurturing their adoptive family was being in their middle childhood and on. Such a child's ability to trust and love would have essentially become "stuck" in infancy, even though the rest of their body continued to grow. The question of whether the critical period idea or the sensitive period idea is more correct boils down to whether this stuckness can be overcome, in full or in part, in the child's later life.
Theorists who support sensitive periods believe that while it will be far more difficult for the child and the child's teachers and caregivers to learn what was not learned during the window of opportunity, these children can still develop the missing capacities and skills later that they did not develop earlier. While some children do seem to get stuck permanently, there is evidence to support the sensitive period idea as well. Some children born in the same understaffed orphanages who are later adopted do go on to learn to love, to trust, and to show affection to their family and friends. In these situations, the families have to have extreme patience and perseverance as they nurture these older children because they are not going to be able to learn that trust and love as fast and as easily as infants.
However, it's also important to remember that critical or sensitive periods can also affect children in other ways than just neglect or deprivation. For example, there is a critical or sensitive period for language acquisition that occurs during infancy. Children begin learning how to understand and create language from the time they're born. They will absorb and copy the language they hear all around them during that critical or sensitive period early in life. However, for many different reasons, children, and adults, may leave their original home and move to a new country or region where people speak a different language. They will need to learn to understand and create the new language, even though they were not exposed to it during that early important period. However, while it will take more time and special tutoring, many children, and adults, can learn a new language proficiently later in life. (different parts of the brain are used for sensitive period learning, vs. later learning, but both can get the job done)