I recently came across this "formula" for finding a birthdate*, when all you have is the death date and age, from say, a tombstone or obituary. If you don't have your date calculator handy (there's usually one built into most genealogy organization programs), you can use this "8870 formula":
Tombstone states date of death is April 11, 1866 and the person was 68 years, 8 months and 21 days old.
18660411 (year month day of death)
- 680821 (age in years, months, days)
- 8870 (subtract the number 8870)
17970720 (birthdate of abt. 1797 July 20)
I've played with it a couple of times, and if your month value is over 12, add one to the year and subtract 12 from the months. Same with the day value - if its over 30 or 31, add one to the month and subtract from the days. This isn't always 100% accurate to the day, but you can get a good idea of someone's birthdate at least!
*There are a few online sources for this formula, and it has been printed in a couple of society newsletters lately as well. I read it in my current issue of Berkshire Genealogist.