Birth dates.

[thegreypigeon]

Is there any particular reason for birth dates to be so vague on census e.g. born about 1866. Were people unsure of their own birthdays ?
Did it not mean much to people in these times ?

[johnnyonthespot]

Is there any particular reason for birth dates to be so vague on census e.g. born about 1866. Were people unsure of their own birthdays ?
Did it not mean much to people in these times ?

Which census are you referring to? US or another country. What year?

In the US, the primary purpose of the census is to determine the allocation of seats in the House of Representatives and - over the years - to help decide where public funds are spent, etc. In order to encourage full participation, the census was deliberately designed to not ask for data outside the scope of its purpose; thus there would be no need to know the exact birthdates of the people being enumerated.

[oilman19]

Also, census records are only as accurate as the knowledge of the declarant, who, quite frankly, might be anyone. The declarant may not have been sufficiently knowledgeable about the people he/she was identifying for the census taker.
In addition, since my ancestors were illiterate, their ages did not seem to be important. They actually recorded ages that were off by as much as 10 years sometimes on birth and death records in Italy.
So, there are many reasons for inaccurate ages on the census which is notoriously an inaccurate document. It is frequently useful as a guide but confirming info is very important.
Hope this helps.