Many industrialized countries, such as the United States, use absolute age, the actual time from the individual's birth. It is indeed paramount to American society, because without precise reckoning and proof of chronological age, an individual cannot vote, apply for a driver's license, or get married. Relative dating, however, is more typical. Relative dating usually means that an individual's age is based on the age of others. Person 1 is older than Person 2.
Some cultures use age sets and grades to establish a member's rank in society or age stage.
Age sets are people associated with each other because they were born within a specific culturally determined period of time. An age grade is the position of seniority of the age set as a whole. Generations are particularly important for many groups of people because they are linked with kinship and marriage-ability.