I was looking for the Kenyan 2009 census data and came across that survey's guide for enumerators (ie, data collectors) in PDF form, here. There's an appendix towards the end -- starting on page 60 of the PDF -- that's absolutely fascinating. Collecting information on the age of a population is important for demographic purposes. But what do you do when a large proportion of people don't have birth certificates? The Kenyan census has a list of prominent events from different regions to help connect remembered events to the years in which they happened.
This may well be standard practice for censuses -- I've never worked on one -- but the specific events chosen are interesting nonetheless. Here's the start of the list for Kirinyaga County in Kenya:
So if you know you were born in the year of the famine of (or in?) Wangara, then you were 100 years old in 2009. Likewise, 1917 was notable for being the year that "strong round men were forced to join WWI".
On the same note, the US birth certificate didn't have an option for mother's occupation until 1960! (That and other fascinating history here. Academic take here.) Also, there are 21 extant birth certificates from Ancient Rome.