Marguerite Joseph, 104, entered her real age when signing up to the social networking site, but was presented with an error message.
After her granddaughter Gail Marlow helped her select her birth year as 1908, Facebook automatically changed it to 1928.
Joseph has stayed at 99 years old on the site for the last two years due to the bug.
Her granddaughter updates the Facebook page for her and replies to all the messages on her behalf. Her grandmother mostly uses the site to stay in contact with relatives across the US and Canada.
Marlow told WDIV-TV that she has sent messages to Facebook's Mark Zuckerberg to help fix the issue, but has not received a reply.
A Facebook spokesman said: 'We've recently discovered an issue whereby some Facebook users may be unable to enter a birthday before 1910.
"We are working on a fix for this and we apologise for the inconvenience."
The granddaughter added: "I would love to see her real age on Facebook. I mean, in April she's going to be 105. It's special."