Four in five 11- to 17-year-olds around the world are not taking enough physical exercise, according to the first such analysis, reports BBC.
The World Health Organization says children's health is being damaged as well as their brain development and social skills.
It says failing to take the recommended hour a day of exercise is a universal problem in rich and poor countries.
Boys were more active than girls in all but four of the 146 countries studied.
What exercise counts?
Pretty much anything that makes the heart beat more quickly and the lungs breathe harder.It could include:
The target is 60 minutes of moderate-to-vigorous exercise a day.
"I don't regard it as a ridiculous target," the WHO's Dr Fiona Bull says.
The main reason is health, both now and in the future.
In the short term, being active means:
fitter heart and lungs
stronger bones and muscles
better mental health and wellbeing
Are any countries doing better than others?
The World Health Organization report found inactivity was a universal problem, from Afghanistan to Zimbabwe.
Bangladesh had the lowest levels of inactivity but it still had 66% of children (two in three) not taking an hour a day of exercise, according to the study.
Boys in the Philippines (93%) and girls in South Korea (97%) were the most inactive in the study.