A generation, for purposes of demographic and historical studies, is a group within a population born within a certain period. This period is estimated to be between twenty and thirty years. The basis of determining a generation are different. Some peg it on the duration a population takes to double itself, the average age between parents and their offspring or the age at which a definite group in a population is expected to begin to reproduce.
Globally, since 1900, generations are as follows: