Extracts from 1841 census, under the heading 'DIRECTIONS'

The following guidelines for the Enumerator, might be worth bearing in mind when, on checking the census, the ages of a probable ancestor do not tie up.

"Age and Sex" - Write the age of each person opposite to the name in one of the two columns headed "MALES" and "FEMALES", according to the sex. 

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 of the term of 5 years within which the age is.

Thus - for Persons age:

15 years and under 20 write 15
20 years and under 25 write 20
25 years and under 30 write 25
30 years and under 35 write 30
35 years and under 40 write 35
40 years and under 45 write 40
45 years and under 50 write 45
50 years and under 55 write 50
55 years and under 60 write 55
60 years and under 65 write 60
65 years and under 70 write 65
70 years and under 75 write 70

and so on up to the greatest ages.

If no more can be ascertained respecting the age of any person than that the person is a child or is grown up, write "under 20" "above 20" as the case may be.