Some pupils sitting GCSEs in Northern Ireland have the reading age of a 13-year-old, a survey says.
Secondary school children are not reading challenging enough books and lag behind their primary counterparts in the rate of progress made, according to a study by education provider Renaissance UK.
The Education Department said its literacy and numeracy strategy, Count, Read: Succeed, set challenging targets for attainment in literacy and numeracy which are on track to be met.
The What Kids are Reading report's authors said: "The Renaissance study highlights a persistent problem in Northern Ireland and across the UK in encouraging secondary school pupils to read challenging and age-appropriate books."
Book difficulty levels rise sharply in each year of primary school, meaning that primary pupils are typically reading more advanced books for their age.
The report added: "However, this progress stops when pupils reach secondary school and the difficulty of books read falls as pupils get older."
This decline exists among boys and girls.
"Consequently, the gap between reading ability and pupils' age grows every year of secondary school."
A total of 60,438 Northern Irish young people's reading habits were studied.
The review was written using Renaissance's data and analysed by University of Dundee academic Professor Keith Topping.
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, it said.
Renaissance UK managing director Dirk Foch said: "The fall in teenagers' reading ages is striking."
He said many could even struggle to understand their exam papers.
"This could have a significant impact on their future academic success," he added.
An Education Department statement said Accelerated Reader, the commercial programme on which the 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."
At Key Stage 3 it is a statutory requirement for pupils to be given opportunities to become critical, creative and effective communicators by reading and viewing for key ideas, enjoyment, engagement and empathy.
It added: "The department is currently engaged in a range of activities, both within and outside school, which support teachers and school leaders in the holistic development of literacy skills.
"This includes support in the early years as well as additional help for schools.
"A new parental engagement campaign, Give your child a helping hand, is running across a range of media at present."