The X Factor is a book.
The X Factor (UK series 7) . Following bootcamp , successful acts were split into four categories : Boys ( male soloists aged 16 to 28 ) , Girls ( female soloists aged 16 to 28 ) , Over 28s ( soloists aged 29 and over ) and Groups . This was the first series of the show to be filmed in high definition , and was simulcast on ITV1 HD and STV HD . It was the most watched series to date , with an average of 14.13 million viewers per episode .