It is not clear what you really need. Is it for a general knowledge or for some specific detail? By saying infancy nature do you mean some technical term or just a baby.
What I give below is a general answer for common purposes.
In olden days children who have attained puberty and physical maturity were considered adults. However the modern social concepts give a longer period between childhood and adulthood, called adoloscence. So adolescence is actually a socially originated concept. It is a socially considered transit period. As such there is no exact boundary at the lower level. It can be anywhere from 10 onwards though as a norm it is taken as from 12 to 18 years which is also known as entering into teen ages.
The word adolescence originates from the Latin root word ' adolescere' which means 'to grow up'. So adolescence is a stage of growing up from child stage to adults. It is a period of fast growth in body and mind of the child, from the onset of puberty to adulthood. The psychological changes added with the physical changes make it not to accept it as inferior to adults.
There are specific scientific literature on the physical changes. The psychological changes mainly manifest as questioning, refusal to accept the stereotype justifications and reasoning by adults and trying to assert individual rights. It is a passing stage and stops when the adulthood is reached.