One of the outstanding characteristics of the third-world countries is the Adolescence of their population, referring to the age group below 15 years old. Population Adolescence is, by itself, regarded as a complex social problem in the third-world countries. This problem is manifested in different dimensions both at present and in future; furthermore, it brings failure or deficiency to many future plans.The age composition of population is related to the degree of development in different countries. This means, the more a country is developed, the more its population below 15 years old decreases and the more it is added to the population of the other two groups. The founder of this idea is Wertheim, the Dutch demographer and sociologist. In his view, at least 40 percent of the underdeveloped countries population are below 15 and accordingly the test of %40 or Wertheim test has been introduced.In this study, two indices of Wertheim and Vernet have been implemented to identify Adolescence in Iran's population. Statistics are based on 1956-2006 censuses. Iran's population has turned to be adolescent within 1956-1966. The Adolescence intensity has increased in 1976-1986 interval having a more logical rate within 1966-1976 period.Eventually, this intensity has gradually decreased in 1986-1996 having a more decrease rate within 1996-2006 interval.The Adolescence or agedness of population, in principle, causes diverse problems in respect to social and economical planning especially in national investments. About %50 of adolescent population in a thirdworld country are just consumers having no roles in production and its increase rate. In adolescent populations the death rate in children is so high and life expectancy is considerably low. The adolescent composition of a given population needs vast investments to provide the age group below 14 with sanitation, education, welfare and housing.