Basically, people (especially teens and
adults) who read, watch or play something that's not really intended for their actual age demographic. One example is Harry Potter which was originally intended for children and
adolescents but quickly
caught on with adults as well.
Hank the Cowdog was originally written for
adults, but became incredibly popular with children. That's one example of the
periphery demographic stereotype reversed (the stereotype being that most think that adults always read something originally intended for kids, but now it's kids reading something originally intended for adults).