In 1900, the Mexican population was 13.6 million. During the period of economic prosperity that was dubbed by economists as the "Mexican Miracle", the government invested in efficient social programs that reduced the infant mortality rate and increased life expectancy. These measures jointly led to an intense demographic increase between 1930 and 1980. The populations annual growth rate has been reduced from a 3.5% peak in 1965 to 0.99% in 2005. While Mexico is now transitioning to the third phase of demographic transition, close to 50% of the population in 2009 was 25 years old or younger. Fertility rates have also decreased from 5.7 children per woman in 1976 to 2.2 in 2006. The average annual population growth rate of the capital, the Mexico City, was the first   in the country at 0.2%. The state with the lowest population growth rate over the same period was Michoacán (-0.1%), whereas the states with the highest population growth rates were Quintana Roo (4.7%) and Baja California Sur (3.4%), both of which are two of the least populous states and the last to be admitted to the Union in the 1970s. The average annual net migration rate of the Federal District over the same period was negative and the lowest of all political divisions of Mexico, whereas the states with the highest net migration rate were Quintana Roo (2.7), Baja California (1.8) and Baja California Sur (1.6). While the national annual growth rate was still positive (1.0%) in the early years of the 2000s, the national net migration rate was negative (-4.75/1000 inhabitants), given the former strong flow of immigrants to the United States; an estimated 5.3 million undocumented Mexicans lived in the United States in 2004 and 18.2 million American citizens in the 2000 Census declared having Mexican ancestry. However, as of recent years in the 2010s, the net migration rate reached 0, given the strong economy of Mexico and a weakening American economy, causing many of its former residents to return. However, Mexico is the second country in the total number of immigrants to the United States from 1830 to 2000, after Germany.
Question: How many years was the time span where Mexico had an intense demographic increase?
Answer: 50