University of Dundee academic Professor Keith Topping carried out a study of the reading habits of about 26,000 Scottish students for online assessment provider Renaissance UK.
He found that primary-school students typically push themselves to read more complex books than secondary-school pupils.
By the age of 11, a pupil’s reading age falls behind their chronological age by one year and when they reach 12-13 their reading age has fallen behind by two years.
When pupils hit 16, their reading age typically falls to at least three years behind, the study found.
This means that many 16-year-old students sitting exams this year will have a reading age of 13, researchers said.
Prof Topping, who specialises in educational and social research at the university’s school of education, said: “The brain is a muscle that literacy skills help train. As it gets more toned, like all muscles, it needs more exercise.
“Currently, primary schools are exercising it more vigorously by reading more challenging books - we now need to replicate this in secondary schools.
“More discussions between young people about books they are reading should be encouraged.
“I would also encourage all secondary teachers, not just English teachers, to look closely at their pupils’ literacy levels and remember - even the brightest students need to be stretched.”
The findings have been released as part of Renaissance’s 2017 What Kids Are Reading report, which found that the trend was reflected across the UK.
Renaissance managing director Dirk Foch said: “Most primary schools place a large emphasis on developing literacy skills.
“However, this is rarely transferred onto secondary school and, as a result, literacy standards at secondary level are a persistent challenge.
“It is clear that if we are to avert a slide in literacy standards many secondary-school pupils would benefit from dedicated reading-time in secondary school - as demonstrated in another study by Professor Topping, which shows that students make the most progress when they successfully read suitably-challenging books for at least 15 minutes per day.”