It’s a mystery to me why UK censuses have ever asked for people’s age. Age is variable, and requires arithmetic. Birthdate is fixed, and requires merely memory. And, without the memory of birthdate, it is not possible to compute age.

The original use of the census was (I seem to remember) to keep track of how many military-age men were present. It doesn’t seem harder to tell the enumerator, “Count all the men born between 1801 and 1825,” than it is to tell them, “Count all the men aged between 16 and 40”.

Was it really possible that people in the 1800s really had a better idea of their age than they did of their birthdate? It sounds unlikely.

Another possibility is that the designers of the census were wary of rousing antagonism because of the invasion of privacy. Censuses always risk this. The 1841 census asked adults their age rounded down to the nearest 5-year multiple (!). The only reason I can think that this was done was as a sop to privacy campaigners. I haven’t seen any detailed discussion of this on the web, though.

Anyway, UK census returns that are public (1911 and earlier) all show age rather than birthdate, with the added complication of 1841 returns showing rounded age. Researching family history, we want to know birthdate. So, the birthdate calculator includes an option for calculating earliest and latest possible birthdates from the age stated on a UK census return.

Here’s a picture of an actual 1911 census return, showing how confused the original data often was. That was written by my grandfather. Most of the ages are wrong, and I have the birth certificates to prove it. It’s very common for ages in census returns to be wrong – some of my relatives couldn’t write, let alone add up – but the birthdate calculator gives the precise limits on birthdate assuming that the data is correct.

There is confusion, at least in my mind, over whether the age entered on a census form should be that on the Sunday of the census, or that of the next day, the Monday. I suspect the rule may never precisely have been set down. The birthdate calculator makes the assumption that the age on the Sunday was stated. Maybe I should add an option to make it Sunday or Monday! Since the ages given for some of my relatives were out by 3 years, I’m not sure that Sunday or Monday matters too much.

The birthdate calculator makes allowance for the fact that, in the 1841 census, an age given which is both a multiple of 5 and is 15 or more, could have been rounded down. The calculator gives a wider range of possible birthdates for such ages.