  The Health State Function theory is applied to find a quantitative estimate
of the Human Development Stages by defining and calculating the specific age
groups and subgroups. Early and late adolescence stages, first, second and
third stages of adult development are estimated along with the early, middle
and old age groups and subgroups. We briefly present the first exit time theory
used to find the health state function of a population and then we give the
details of the new theoretical approach with the appropriate applications to
support and validate the theoretical assumptions. Our approach is useful for
people working in several scientific fields and especially in medicine,
biology, anthropology, psychology, gerontology, probability and statistics. The
results are connected with the speed and acceleration of the deterioration of
the human organism during age as a consequence of the changes in the first,
second and third differences of the Health State Function and of the
Deterioration Function.
  Keywords: Human development stages, Deterioration, Deterioration function,
Human Mortality Database, HMD, World Health Organization, WHO, Quantitative
methods, Health State Function, Erikson's stages of psychosocial development,
Piaget method, Sullivan method, Disability stages, Light disability, Moderate
disability, Severe disability stage, Old ages, Critical ages.
