Why were ages on 1800s census's so loosely accurate?

[Paulo Leeds]

I often see ancestors with 1,2,3 years difference each census.

lies? or did they just not know exactly how old they were/year they were born??

[CaroleW]

You need to remember that there were very high levels of illiteracy back then and people could not always "do their sums" when it came to ages

[dawnsh]

and they didn't have to know when they got to 60 or 65 as there wasn't the old age pension then 

Ages weren't really that important

[Paulo Leeds]

i guess ages weren't the wholly documented thing they are now, every 10 years people probably 'forgot'

it's easy to forget that life without bureaucracy, departments, systems, computers was so much different

[Mart 'n' Al]

Women have lied about their ages for centuries!

Martin

[Maiden Stone]

1. Ages of adults on 1841 census wasn't intended to be accurate. Instruction to census enumerator regarding ages was to round down age of each adult to a multiple of 5. A child's age (i.e. under 15) was to be entered as given.
"Write the age of every person under 15 years of age as it is stated to you. For persons aged 15 years and upwards write the lowest term of 5 years within which the age is.
Thus for Persons aged
                              15 years and under 20 write 15
                              20 years and under 25 write 20
                              .....
and so on until the greatest age.
If no more can be ascertained in respect of any person other than that the person is a child or grown up write 'under 20' or 'above 20' as the case maybe."

www.hunimex.com/warwick/census/1841_cnum_inst.html

Some census collectors followed this instruction, some didn't.

2. 1841 census was taken in July, subsequent census days were in March or April.
Therefore a person who had turned 9 years of age in May or June 1841 would have been recorded as 18 years old on 1851 census and 28 in 1861, providing their exact age was known.

Edit. Oops! Just noticed and corrected an error in the age ranges.
 

[philipsearching]

Women have lied about their ages for centuries!

Cheeky. 

Before the system of registering births was introduced the most accurate records are in family bibles (assuming the family was literate and could afford luxuries like bibles).  Some baptism records mention a date of birth, but this is a small minority.  Middle-class and upper-class families would have been more likely to remember and celebrate birthdays as they had a higher standard of living than the working-class struggling to survive.
The UK state pension for people over 70 began in 1909 (over 70 years after the registration system began).  Until then the only age limits were for marriage, employment and military service - but I doubt that people were always required to provide proof of age.

Philip

[Nick_Ips]

I often see ancestors with 1,2,3 years difference each census.

lies? or did they just not know exactly how old they were/year they were born??

I would say you are quite fortunate if you've only encountered differences of 1, 2 or 3 years. 

As well as people being 'economical with the truth' (especially popular with older women marrying younger men) you also have to bear in mind that 1841-1901 the records we are looking at have been copied at least once.

So on top of deliberate inaccuracy and unintentional inaccuracy, we are also potentially seeing the results of simple misreading/miscopying of what was originally written.

And in some cases a +/-1 year difference might be as simple as having a birthday around the time the census was taken, and therefore having a birthday before - or after - census day, depending on the decade.

[Ayashi]

I have some ancestors who were consistent and some who weren't so much. I have one ancestor who, according to the census, was born between 1803 and 1821...

I think sometimes they tailored age to their spouse. The man above had a first wife who was older than him and his age was higher there, but then remarried to a woman much younger and lo and behold his age drops. This was probably more common with women though.

With children, it's possible that, especially with a lot of children, the parents couldn't quite keep track of the ages.

As others have said, however, ages weren't as marked with milestones back then.