It admits little is known about how children transmit the virus but cites evidence that teenagers can infect others in the same way as adults.
Children aged five and under should not normally wear masks, the WHO said.
More than 800,000 people have now died with coronavirus worldwide.
At least 23 million cases of infection have been registered, according to Johns Hopkins University, with most of them recorded in the US, Brazil and India.
However the true number of people who have had the virus is believed to be far higher, due to insufficient testing and asymptomatic cases.
The numbers have been rising again in countries as diverse as South Korea, EU states and Lebanon.
WHO chief Tedros Adhanom Ghebreyesus has said he hopes the pandemic will be over in two years but a top scientific adviser in the UK warned Covid-19 might never be eradicated, with people needing regular vaccinations.
