The year 1921 is taken as the demographic divide for the reason that before this year, the population was not stable, sometimes it increased and at other times it decreased.
The growth rate of population was generally low before 1921. But after this year, there has been considerable and continuous increase in the population.
Between 1901 and 1911, the total increase in population was 5.9% and between 1911 and 1921, there was a decrease of 0.39%. In the decade of 1921-1931, the increase was 11.1%, from 1931 to 1941, it was 14.00% and during 1941-1951, it was 13.5%. It is going increasingly since then. Hence the year 1921 is rightly called the demographic divide.