This book was written as a series of brochures to make available to singles. The brochures are limited to what will print on two sides of a sheet of paper, so few references to primary sources (except Scripture) are given. Some people have asked for a list of the best sources.
Grace Wyshak and Rose Frisch wrote “Evidence for a secular trend in age of menarche in the New England Journal of Medicine 1982, Volume 306, pages 1033-1035. They reviewed 218 studies in Europe and the USA covering 220,037 women conducted between 1795 and 1981. They found that the age of a woman’s first menstrual period (related to puberty) declined greatly during those two centuries. This journal is available in many larger libraries, and a brief summary is in the New York Times at http://www.nytimes.com/1982/05/11/science/science-watch-earlier-menstruation.html .
Robert Bremner edited a five-volume series of primary sources related to the changes that took place in the status of youth in America during the time that adolescence was invented. The first two volumes are a great source of material actually written while it was taking place.
Bremner, R. H. (Ed.). (1970) Children & youth in America: A documentary history. Vol I: 1600-1865. Cambridge, NY: Harvard University Press.
Bremner, R. H. (Ed.). (1971) Children & youth in America: A documentary history. Vol II: 1866-1932. Cambridge, NY: Harvard University Press.
The best sources of data are usually those gathered by the government. In the USA the U. S. Census Bureau has a wide variety of statistics about many things at http://www.census.gov/. The most valuable of these over the years since 1878 are the annual editions of the Statistical Abstract of the United States at http://www.census.gov/compendia/statab/. These are available online free of charge. In the UK the best source is the Office for National Statistics at http://www.statistics.gov.uk/hub/