Teenagers are selecting "easier reads" in their book choices, rather than more challenging classics, according to a survey published for World Book Day.
A study of reading habits of 300,000 pupils in 1,600 UK schools found many 13- and 14-year-olds opting for books with a primary school reading age.
Boys were particularly likely to read books which were less difficult.
Report author Prof Keith Topping says this could explain why teenagers' reading levels could "lag behind".
The average reading age of the books chosen by 13 and 14-year-olds was only 10 years, claims the report.
The study looked at the types of fiction and non-fiction children were choosing to read, such as books borrowed from a school library, public library or books they owned themselves.
"From the start of secondary school, young people, including both high ability and struggling readers, tend to read books that do not challenge them enough," said Prof Topping, from Dundee University's school of education.
He says that without guidance from teachers and librarians there seems to be a "slump" in the reading levels of books chosen by teenagers, who appear disinclined to try more complicated works.
Prof Topping also warns that by not engaging with more sophisticated books they fail to engage with bigger and more complex ideas: "They're not only not reading at a higher level; they're not thinking at a higher level."
He also questioned whether teenagers had a sufficient breadth in the type of books they were reading.
Boys were found to be reading at a lower level than girls - but the What Kids Are Reading survey, which has been running for five years, says the gap has been narrowing.
The report suggests that teenage boys are in particular need of encouragement to read more stretching books, to prevent them from drifting towards younger books.
The study, published by education software company Renaissance Learning, also looked at a younger group in primary schools.
In contrast to secondary school, primary school children were found to be choosing books which had a reading age above their own.
Pupils aged seven and eight had chosen books which an average reading age of eight years and 10 months.
This might reflect the fact that for younger children, parents and teachers might intervene more in reading habits.
Barbara Band, vice-president of the Chartered Institute of Library and Information Professionals, said the report showed the importance of "guiding children to the right reading".
She said: "A good library should be at the heart of every secondary school; teaching research skills, supporting learning across the curriculum and helping young people develop a lifelong love of reading."
World Book Day is an international day to encourage reading, promoted by the United Nations Educational, Scientific and Cultural Organisation (Unesco).