Guest Boot Camp judge Nicole Scherzinger apparently persuaded Simon Cowell and Louis Walsh to make the change because the quality of the older singers was so high.
A source told The Sun: "There are normally twice as many under-25 singers as over, but with some of the best acts being in their mid-20s this year, they wanted to make sure they didn't lose them."
An X Factor spokesman confirmed: "At Nicole's suggestion, and agreed by Simon and Louis, the boys' and girls' category age limit has been increased to 28.
"The judges believe this will strengthen the competition and talent."
In addition, there will now be eight acts progressing to the Judges' Houses stage of the show, instead of the usual six.
It is also rumoured that four acts from each category will move on to the live shows, creating a record total of 16 finalists.
> 'X Factor' hopefuls cause chaos in hotel