Couch potato teenagers do as little exercise as 60-year olds, according to new research.
The study suggests physical activity among children and adolescents is even lower than previously feared - fuelling the growing obesity epidemic.
Three in four girls aged 12 to 19 - and more than half of boys - are failing to meet guidelines recommended by health experts.
Professor Vadim Zipunnikov, of Johns Hopkins Bloomberg School of Public Health in the US, said: “Activity levels at the end of adolescence were alarmingly low, and by age 19, they were comparable to 60-year-olds.”
Half of six to 11-year-old girls - and over a quarter of boys - are also falling short, according to the study.
Prof Zipunnikov said: “For school age children, the primary window for activity was the afternoon between 2pm and 6pm.
“So the big question is how do we modify daily schedules, in schools for example, to be more conducive to increasing physical activity?”
He warned the teen years represent highest risk - with many being so idle they could be compared with an adult 40 years older.
Increases in activity levels were only seen in 20-somethings, possibly because they are just beginning work.
Activity decline at 35
Starting at age 35 activity levels declined throughout midlife and older adulthood, reports Preventive Medicine.
The study followed 12,529 participants in the US who wore tracking devices for seven straight days - removing them only for bathing and at bedtime.
They measured sedentary, light or moderate-to-vigorous physical activity and the findings were broken down into children (six to 11), adolescents (12 to 19) as well as 20 to 29 year olds, 31 to 59 year olds and 60 to 84 year olds.
In the UK a quarter of boys between two and 19 and a third of girls are overweight or obese.
The cause has been blamed on our modern lifestyles, including our reliance on the car, TVs, computers, desk-bound jobs and high-calorie food.
The NHS says obesity levels have more than trebled in the last 30 years and, on current estimates, more than half the population could be obese by 2050.
Prof Zipunnikov said the patterns could inform programs aimed at increasing physical activity by targeting not only age groups but times with the least activity - such as during the morning for children and adolescents.
For all age groups, males generally had higher activity levels than females, particularly high-intensity activity.
But after midlife, these levels dropped off sharply compared to females. Among adults 60 years and older, males were more sedentary and had lower light-intensity activity levels.
One hour of exercise per day recommended
The World Health Organisation recommends at least 60 minutes of moderate-to-vigorous physical activity a day for five to 17 year olds.
But the researchers say there is a growing consensus for the benefits of reducing sedentary behaviour and increasing even low-intensity levels of physical activity.
Prof Zipunnikov said: “The goal of campaigns aimed at increasing physical activity has focused on increasing higher-intensity exercise.
“Our study suggests these efforts should consider time of day and also focus on increasing lower-intensity physical activity and reducing inactivity.”
The findings follow research earlier this week that showed three in ten people across the world are overweight - 2.2 billion of the planet’s population.
They are putting themselves at increased risk of cancer, heart disease and diabetes.
Scientists said it represented “a growing and disturbing global public health crisis.”