Developmental psychologists have tended to divide up the human life span into the following three categories. Most books on human psychological development use this breakdown.
One can portray human development as a series of hurdles of tasks that the person must jump over or master. For example in childhood the child has to learn to walk, talk, be toilet trained, deal with the oedipus complex, go to school and so on. They need to go through all the stages of cognitive development and moral development and so on. A child goes through a period of significant and quite fast physical change and psychological change.
One can say the same applies for the teenager. They have a whole series of hurdles to jump or tasks to master. They also have to go through a series of cognitive and moral development stages, learn how to engage with the opposite sex, develop a social support system with their peers and so forth. Like in childhood they go through a period of quite fast and significant physical and psychological development.
Developmental psychologist have decided to make a separation between the stages of childhood and adolescence. This corresponds with the age about when puberty starts. For some reason at this point they say a person has moved onto a new stage of psychological development. They distinguish between the psychological stage of childhood and the psychological stage of adolescence as occurring when puberty arrives.
To my mind this is an arbitrary distinction which is a nonsense and I have not heard of the rational for doing this. One could argue that this distinction needs to be made because the developmental tasks required in childhood are significantly different than the developmental tasks required in adolescence. This is a hypothesis with which I would agree. Without a doubt a 4 year old child is dealing with very different developmental issues than a 14 year old teenager. However the choice of 12 or 13 years of age for the new developmental stage to arrive seems quite a random decision to make.
It can also be argued that a 2 year old child and an 8 year old child are very different physically and the psychological developmental issues they are dealing with are also quite different. So why not conclude that they are two separate developmental stages instead of including them in the same developmental stage as is currently the case. The same of course applies for a 13 year old and the 18 year old. At the moment they are in the same developmental stage but they are also quite different physically and psychologically. Why are they not defined as two separate developmental stages?
One can really see the two stages of childhood and adolescence as one developmental stage. In both stages the young person is going through significant and rapid physical and psychological change. Whereas in adulthood this is not the case. Yes there is indeed physical and psychological change occurring in adulthood but it can be seen as much slower and less psychologically significant. Thus there is much more focus on the younger person as this is where more of the difficulties can occur if the development is not occurring as it should. Developmental psychology is about developmental change in the person so one could see the human life cycle in a different way. Divided up into phases where there is a lot of psychological change occurring and not.
In this model pre-adulthood (childhood and adolesence) is seen as a phase of significant physical and psychological development. Adulthood is a phase of much less significant physical and psychological development. At the end of life we have another developmental stage, Post-adulthood. One could say this stage begins in the late 50s or 60s. In this period there is significant physical change as the body again moves into such a phase of change. These changes can have significant consequences including major disability, deformity or death. These changes force the person into significant psychological change as they accept, or seek to deny, the consequences of such change and acceptance of their bodily changes. With the physical changes the person has to change their perception of them self and their interaction with others and the environment. Thus one could argue that this constitutes a significant level of psychological change and hence we have another period of the development cycle.
For instance the loss of sight, hearing, memory, physical abilities and many other conditions like Parkinson's disease all require the person to make a significant psychological readjustment about how they perceive them self, their identity and how they interact with the world. Of course with illness and accidents these can occur in the adulthood stage and also result in disability and or disfigurement. Again the person has to change their perception of self and the world like happens in the Post-adulthood stage.
This model is about periods of developmental change and periods of non significant change which seems to be the task for the developmental psychologist. In the beginning of life there is a period of significant physical and psychological change. To arbitrarily break this up seems to be a nonsense. This is followed by a period of about 30 to 40 years where there is much more consistency in ones physical and psychological state such that much less psychological development and change occurs. Finally in the last couple of decades of life one again goes into a state of significant physical change. It is postulated that this would then result in significant psychological change as the person reorients their identity and perception of self to correctly align with their physical self.