A survey of more than 60,000 children here has found that many are not reading books which are challenging enough for their age.
The UK-wide study by literacy and assessment provider Renaissance UK, which surveyed 60,438 young people from year 1 to year 11 in Northern Ireland, found that book difficulty levels rise sharply with each year of primary school, but this falls off at secondary school.
The study found the decline was consistent among both boys and girls.
The results, which are detailed in the What Kids Are Reading Report released today, include findings that children in the first year of secondary school tend to be reading one year less than their chronological age.
By the second year (age 12) pupils are reading two years below their chronological age. And by the third, fourth and fifth year of secondary school pupils are reading at least three years below their chronological age - meaning that many pupils sitting their GCSEs at age 16 have the reading ability of a 13-year-old or lower.
The slump was also found in schools across the UK.
The most read book in Northern Ireland's primary schools is Roald Dahl's classic The Twits, followed by comedian-turned-author David Walliams' books The Midnight Gang and Gangsta Granny in second and third place.
In secondary schools Gangsta Granny takes the first spot, followed by Jeff Kinney's Diary Of A Wimpy Kid and then, again by Walliams, The Midnight Gang. Dundee University's Professor Keith Topping described the findings as striking, and said pupils should be encouraged to push themselves to read more difficult books.
The Department of Education said: "The department looks forward to seeing the report and will give it due consideration in the context of our literacy and numeracy strategy, Count, Read: Succeed, which sets challenging targets for attainment in literacy and numeracy and which we are on track to achieve.
"Accelerated Reader, the commercial programme on which this study is based, focuses on encouraging children and young people's personal reading and as such is not part of structured classroom learning.
"While the outcomes provide useful data, classroom teachers take a much broader approach to improving literacy and to addressing reading challenges through a broad suite of interventions, well-informed support and a thriving school library provision."