I have been very fortunate in that the vast majority of my ancestors' birth dates, as reported in the death records matched the dates from other sources. Family Bible records, military records and pensions, compiled genealogies, county history biographies, census enumerations (especially the 1900 census which gives the month and year of birth for each family member), cemetery records, obituaries and, when available, church baptisms or birth certificates have all proven to be invaluable sources for locating the date of birth of ancestors.
What happens when the tombstone inscriptions, death registrations, obits, and other available records don't agree? Which records are correct? Do you have any primary sources available? What if your ancestor died before the month and year of birth were given on that 1900 census? Is that 1900 census information correct?
This was the problem I ran into with my maternal great-grandfather's family--each and every one of his siblings! His parents' dates of birth and death matched in all of the records and so have those of his children and spouse. His and those of his brothers and sisters disagreed in almost every source checked!
Two McHugh brothers came to America from County Donegal, Ireland during the early 1830s. They spent about ten years working in the Pennsylvania coal mines before heading west for Galena, Illinois and finally settling in Lafayette County, Wisconsin. John McHugh's marriage to Sarah Hickey in Pennsylvania has not been located, nor have the births of the children born before the family reached Wisconsin. Thus begins the mystery.
John and Sarah had six children. Death registrations (county and/or Catholic Church), or tombstone inscriptions exist for five of them. Unfortunately, most of the death registrations don't give birth dates. Daniel, the eldest, in most records is given a birth year of 1837. His tombstone gives the dates 1837-1907. William died in 1906 at age 52. His tombstone reads 1841-1906, that's 65 years! James died in 1898 at 50 years of age. The dates on the tombstone match (1848-1898), but the church lists his age at death as 53. Mary Ann died in 1903 at 53 years of age, the information inscribed on her marker. John died in 1904 at age 52. His grave marker shows 1853-1904 (50 or 51, not 52.) Charles, the youngest, took off for the western gold fields and was never heard from again.
The census records tell a different story and are very consistent. According to the 1870 census, Daniel was born about 1841/2, William about 1843/4, James about 1845/6, Mary Ann about 1848/9, John about 1850/1, and Charles about 1852/3. The 1850, 1860, and 1880 censuses match up within a year. The 1900 census gave William's birth date as February 1847, John's as December 1850, and Mary Ann's as December 1852. Nothing turned up for Daniel or Charles. My great-grandfather, James, had died two years earlier.
Fortunately, John and Charles were born in Lafayette Co. and their births and baptisms were recorded by St. Matthew's Catholic Church. John was born in December of 1849 and baptized in May of 1850. Charles was born in December 1852 and baptized in May of 1853. Therefore John was 52 when he died. In most cases the earlier census records were more accurate than the available death records, obituaries and tombstones.
What do you do when there is such a discrepancy in the dates of birth of a family group? Go with the most consistently reported date if the actual date isn't available. Be sure to record the other dates and document the sources for each one. One question remains, what DID happen to Charles after he headed west?