Added this explanation of ages in the 1841 census to the Census Info page, under Understanding Your Results:
In the 1841 census, the age of persons over 15 was supposed to be rounded down to the nearest multiple of 5. For example, a person aged 19 would be listed as 15, a person aged 22 would be listed as age 20, and a person age 59 would be listed as 55. In practice, many census officials either did not round down at all or only rounded down for higher ages, such as over 20, or (less frequently) rounded down ages below 15. In general, the age of a person under 15 is probably accurate to within a year or two. For persons over 15, any age that is not a multiple of 5 is likely also to be accurate – for example, if a person is listed as 27, he or she probably really is 27 or thereabouts, rather than 25. The area you have to be careful of is persons over age 15 whose age is a multiple of 5 – they may be up to 4 years younger than their census listing shows – so if your ancestor is listed as 50, remember that he or she is likely actually between the ages of 50 and 54. This, of course, does not even take into account the errors made by census officials and family members reporting the ages of others ! Finally, if the age of a person was unknown, children were supposed to be recorded as “under 20″ and adults as “over 20″.
For professional trades, the following abbreviations are used in the census: ‘J’ is for journeyman, ‘AP’ for apprentice, and ‘SH’ for shop man.