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Calculating Birth Year or Birth Date from Age
There are several kinds of records that give us a person's age in a particular year, the most common and obvious being the Federal censuses, so we are often faced with determining a birth-year from this information.   If I can make no other point on this page than this one, it is that:
 
Any record providing a person's age in a particular year will always produce TWO possible birth years, depending on whether the person has or has not had their birthday yet that year.  The widespread practice of simply subtracting the age from the year to produce a single birth year is incorrect.

For example, if someone is age ten in the 1850 Census, it means they could have been born in 1840 or in 1839, depending on whether or not they had had their birthday by the time they were enumerated.  The correct practice is not complicated.  Just subtract the age from the year, as you have always done, but then include the next earlier year as part of an age range.

If you know the date (year, month, and day) when the person's age was taken, but the age is given only in years, the calculation is still the same.  That is, knowing the precise date the age was taken (e.g., as in knowing the age on the date a marriage license was obtained) doesn't change the calculation.
If you know the date (year, month, and day when the person's age was taken, and you know the age in years, months, and days (as one often does from  a tombstone incription), then the best way to calculate the birth date is to use Ben Buckner's JavaScript Birthday Calculator.  This derived date may not be exact, however, because you don't really know if the person who originally calculated the age did so correctly or used the same method (there's more than one way to do it). 
One reason it's important not to imply that you know more than you do as in using just a single birth year from census data is that by doing so you may mislead someone, including yourself!

For example, if you say your James Brown was born in 1832, instead of 1831/2, someone looking for a James Brown born in 1831 may pass you by when, in fact, it was a match just as you, yourself, may be mislead into not looking for a James Brown born in 1831.  Likewise, if you give a calculated birthdate that is off by a few days or weeks, but don't precede it with ca., someone else may have the correct birthdate (e.g., from a family bible), so dismisses your record by failing to see the match when there really was one.  And they failed to see the match because a full date without ca. (e.g., "12 Oct 1731") implies that you had a sound basis for giving it.  In other words, "12 Oct 1731" doesn't look like a guess or an estimate, it looks like a certainty.  Adding ca. at least lets other researchers know that there is some "ify-ness" about the exact date.

The bottom line is 1) to not read more into your data than is there and 2) to always remember a fundamental maxim for any researcher, genealogical or otherwise:

Bad data are not better than no data!

Don't let your desire to "fill in the blanks" lead you to asserting more than what is actually supported by the evidence because:

Misinformation misleads.
©1999 Diana Gale Matthiesen
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