From Fan History Wiki
Demographic surveys, surveys of beliefs regarding fan fiction are frequently done in fandom. Looking at them from a historical perspective can help to guage changes in fandom. While many of these surveys have issues with how they were designed, they are nonetheless useful barometers.
- An informal survey posted on Usenet showed that the people who read fanfic range in age from 12 to 55 (with a particular popularity amongst the twentysomething demographic), ranging from teenagers and university students to professionals by trade. In many cases, fanfic provides a way to perpetuate the memory of a series, to keep the flame alive even after the show has long since been cancelled. Although fanfic is popular with both sexes, it's predominately written by women. Most people tend to discover fanfic by pure happenstance, stumbling across archives while perusing the Web, often soon after getting online for the first time.
One survey was done in 2003 at Montgomery Blair High School by a member of the school's newspaper staff for an article about fan fiction. The survey found that only five percent of students had heard of fan fiction. (https://silverchips.mbhs.edu/inside.php?sid=2136)