1921 Censuses

Ages in years and months can be tricky to calculate, particularly if the individual had yet to celebrate their birthday, and the delay in taking the census may also have caused confusion, since the forms showed the original April date (it still causes confusion for some family historians). There was also the question of whether to always round the number of months down, or whether to round it up if that was closer.

Nevertheless, my experience is that the ages in 1921 are the most accurate of any of the published censuses - though the 1939 Register is generally more reliable.

 

That's not a fair comparison since the published UK censuses don't record dates of birth, they show ages. A better comparison for the 1939 Register dates of birth would be birth register and baptism registers entries, and death register entries (after the change from age to dob).

Most people are better at remembering when they were born than how old they are, since their birthdate doesn't change - which means that arithmetical errors can creep in. However, the person who completed the form could well have the opposite problem when entering other members of the household who were not present. They would most probably know the day and month when their spouse or child celebrated their birthday, and they might well know how old they were last birthday - but if they got the calculation wrong, the year of birth would be out.

In some cases it might be possible to deduce which family member completed the household schedule in 1939!

 

I suspect the error was made by the enumerator when he was copying the entries into the register - you see similar things in the censuses with ages and birthplaces sometimes being switched. The fact that both birthdays were in December makes it more likely that the numbers were inadvertently switched during the transcription process.

I suspect that transcription errors were more likely towards the end of the register, as the enumerator tired of the mindless task, but it isn't something I've ever investigated. Birthdates weren't shown on identity cards so errors like this wouldn't be picked up until NHS cards were issued, and even then only if the individuals were sufficiently concerned to point out the error.