Write a summary of the following text. World Health Organization (WHO) identifies adolescence as the period in human growth and development that occurs after childhood and before adulthood. This phase represents one of the critical transitions in one's life span and is characterized by fast paced growth and change which are second only to those at infancy. Biological processes drive many aspects of this growth and development with the onset of puberty marking the passage from childhood to adolescence. The biological determinants of adolescence are fairly universal; however, the duration and defining characteristics of this period may vary across time, cultures, and socio-economic situations. This period has seen many changes over the past century-puberty for example, comes earlier than before, people marry late, and their sexual attitudes and behaviours are different from their grandparents or even parents. Among the factors responsible for the change are education, urbanization and spread of global communication.