Lazy eye disease can be treated up to the age of 17 in some cases, contrary to common belief, say experts.
Glasses were given to the children who needed them
A US team from the Children's Hospital of Pittsburgh found treatment improved the vision of many older children with the condition known as amblyopia.
Patching the good eye made the muscles of the lazy eye work harder and improved sharpness of vision.
But it is not yet known whether the benefit will be long-lasting, the authors told Archives of Ophthalmology.
They plan to follow the patients into adulthood to find out.
Conventionally, children with amblyopia are treated in their pre-school years. The upper limit for successful treatment has generally been thought to be about nine or 10 years of age.
Dr Richard Hertle and colleagues from the Pediatric Eye Disease Investigator Group recruited 507 children with amblyopia aged between seven and 17 to take part in their study.
All of the children were given glasses if they needed them.
The children were randomly divided so that half would be given treatment for their amblyopia.
This involved wearing a patch over the "good" eye for between two and six hours a day plus activities to make them use their near vision, such as playing computer games or reading.
The researchers followed up all of the children every six weeks for a total of 24 weeks to see how they had responded.
Of the children aged seven to 12, half in the amblyopia treatment group had better sharpness of vision during follow up compared with only a quarter of the children who were given glasses alone if needed.
Of the 13- to 17-year-olds, about a quarter in both the amblyopia treatment group and in the control group had better sharpness of vision during follow up. However, half of the teenagers who had not previously been treated for amblyopia when younger responded to the amblyopia treatment in the trial.
Co-researcher Dr Mitchell Scheiman said: "This shows that age alone should not be used as a factor to decide whether or not to treat a child for amblyopia.
"The opportunity to treat amblyopia does not end with the pre-school years."
Mr John Sloper, consultant at Moorfields Eye Hospital in London, said: "We have always been aware that there are some children who will respond to treatment past the conventional cut off age.
"What this has done is give us a better handle on how many and how well they respond."
He said it was important to pick up and treat cases early to achieve the best outcomes.
Mr Nick Astbury, president of the Royal College of Ophthalmologists, echoed this advice, saying: "Patch as early as you possibly can."