I'm guessing that's how the data are actually gathered for whatever reason. The UN World Population Prospectus has various breakdowns here, and they group peopl

I'm guessing that's how the data are actually gathered for whatever reason. The UN World Population Prospectus has various breakdowns here, and they group people in 5-year groups, so there's a break at 14 and a break at 19. I'm guessing that's where this chart came from. If I had to make an educated guess, I'd say it's probably done this way to make data more comparable between countries for whatever reason. I'm not exactly sure why that would be necessary, but generally when you see something weird like this, it's because something changes a lot from culture to culture, so they use wider buckets to capture everyone who is reasonably comparable. Maybe it's some combination of when different cultures start counting your age and a lack of good birth records in poorer areas, and they figure people can more or less estimate which 5 year block you're in. Another possibility is that the older data sets were collected using some form that had you blocked off in 5 year blocks for whatever reason, and they can't change it now without invalidating the comparability of the data sets. EDIT: If anyone's interested, I found this manual [1] on estimating population age and sex where they go into detail about the assumptions in the five-year age group method. Doesn't really explain why this is desirable, though - I think it might have something to do with how census data are collected in different countries. [0] http://esa.un.org/unpd/wpp/Excel-Data/population.htm http://esa.un.org/unpd/wpp/Excel-Data/population.htm [1] http://www.un.org/esa/population/techcoop/PopProj/manual3/chapter2.pdf http://www.un.org/esa/population/techcoop/PopProj/manual3/ch...