A study by the University of Strathclyde found that after starting primary school children start to become more sedentary.
Children are recommended to achieve at least one hour of high to moderate intensity exercise a day.
But a study of 500 youngsters, who were given wrist monitors, found that by age seven girls did an average of 63.4 minutes of moderate exercise a day, which fell to 56.4 minutes by age nine, 47.4 by 12 at just 41 by the age of 15.
For boys, seven-year-olds were active for around 75.5 minutes, 60.3 by age nine and a steep drop of 51.4 at age 15.
Professor Reilly, from Strathclyde University, said: “Things go wrong when children start school, but that is only half of the day.
“At home, children are sitting or lying down playing electronic games and screen devices, and this trend may even be getting worse. Children are becoming more like mini adults, and sedentary behaviour puts them not only at risk of obesity, but also of diabetes and cancer. Sitting itself is quite harmful,” he added.