Many of us in the U. S. use the Federal census records as one of the backbones of our research. They are great resources. From 1850 forward, every person is named (except for slaves) and their age given, in addition to other details. We need to be a little cautious with the information in censuses, because sometimes a family wasn’t available and the enumerator asked another community member for their information. In addition, we need to be cautious about the age field.
The instructions for the enumerators was to collect the age of each person as of the official census date. For example, June 1 was the official date for many censuses. The enumerator also wrote a date at the top of each census sheet. This is the date when he started enumerating people on that page, not necessarily even when he enumerated the last person on the page (but we can get an idea of dates from the next page’s date, if there is a next page). Was the enumerator careful, asking about the age of each person as of June 1st, or did he just ask their current ages? Unless we can do an in-depth study comparing actual ages to the ones he recorded, we don’t know.
I like to make a two-year birth year estimate from the census. Consider that a person is recorded as 20 years old. I consider that he either just turned 20 or is about to turn 21. This gives us a range: census year – 21 to census year – 20 (e.g. 1859 to 1860, for the 1880 census year). If I am doing more in-depth analysis, considering other records and their evidence of birth date, I then consider the census age, considering that it could be recorded as of the enumeration date or the official census date.
Here is a good article from Family Tree Magazine listing the official census dates for every available U.S. Census: Official Census Dates for US Censuses.