Scenario: Your ancestor gives an approximate birth year of 1870 for himself in all of the censuses during his adulthood. You find a birth certificate that matches him in every detail, but the birth certificate implies he is born in 1861. Can this possibly be him?
The answer: Yes, it can be. Don’t get too hung up on fluctuating birth dates, because people before the 20th century paid little attention to their birthdays and routinely forgot their ages. Some people’s purported ages tended to “drift” over the years, also.
Someone might claim to be 20 years old in the 1900 census, 28 in the 1910 census, 36 in the 1920 census, 44 in the 1930 census, and so on. If they were indeed 20 years old in the 1900 census, then they should be 50 years old in 1930, not 44 years old, right? Yes, but many, many people lost track of their ages and thought they were older or younger than they actually were. Sometimes, people near the ends of their lives could have a vastly different purported age than their actual age (I have seen people lose track of their purported age by as much as 10 years, over the course of their life). Generally, the closer the record is to the individual’s birth, the more likely it is to be accurate. (It’s easy for an 80-year-old to be mistaken for a 70-year-old, but it’s pretty difficult to mistake a 5-year-old for a 15-year-old.)
This was in large part because hard identification documents were not required until partway into the 20th century. Today, we all have physical drivers’ licenses and Social Security cards in our wallets, and we use our birthdays for bank verifications, credit card verifications, etc. In the 1800s and prior, people may have had their birth date written down in a family bible stowed in a drawer somewhere, and that may be the only written record of their birth date a person ever saw. Losing track of ages was also much more common when people migrated great distances. When people stayed in the same little village with all the same people their entire life, everybody tended to remember each other’s ages and birthdays better, and they were usually a short walk away from the church where their birth/baptism record was kept. When they journeyed halfway across the world and settled on a frontier surrounded by complete strangers, there was less reinforcement by multiple people of this knowledge. Another reason ages might have changed could be because young people misrepresented themselves as being older than they actually were in order to qualify for the military, a marriage license, or a driver’s license.
Should we disregard purported ages altogether? Of course not. If there is a discrepancy between the birth dates in two documents, a responsible genealogist will not just ignore the discrepancy. You will have to build a case with other evidence that suggests these are the same people. Do they have the same parents’ names, same birth town, refer to the same siblings, and/or have the same spouse’s name? That can be great evidence that two differing documents refer to the same individual, who just forgot their age. If the day and month of birth is consistent, but the year fluctuates, that can also be an indicator that one person gave multiple differing ages for themselves, because it’s more likely that they would remember their actual birth day and just forget the year. The point is that you can build a case that someone got their birth date wrong, because it happened all the time.