So you want to find your ancestor's birth date. You head out to the cemetery and the only thing is the death date. But, they have listed how old the person is in years, months, and days. There are a few things that must be taken into account when trying to figure out the birth date - leap years, which calendar was being used at the time your ancestor was living, and if you have Quakers which calendar they were using. The Julian and Gregorian calendars are the main two calendars in use. Great Britain and its colonies changed in 1752, dropping 11 days from the month of September. That should confuse us all. Remember not everyone might have followed the new calendar. Also, if you have Jewish ancestry the calendar might be the Jewish calendar used for births and deaths.
Here is are a few sites for a calculator but certainly there are many, many more. Just be sure that they list that they take into consideration things like the leap years, etc.
Good luck in your search.