Teens with braces
The general idea of teens with braces is to touch the memories many people have of the hot girls that they went to school with that had braces. It put them in a certain frame of mind, and makes them feel that the girls are somehow younger. This flies in the face of standard logic, as by law all girls on porn sites are over 18 years of age).
Some site operators will put "fake" braces on their girls to keep them looking young well past their teen years, in an attempt to keep the model in the target market. It is common knowledge that some of the top teen model sites have been open for 4 or 5 years. One site operator brags "Now in her 5th great year as an over 18 teen!" You can do the math and see how this works.
See also: braces