Sitting is replacing physical activity from the time children start school, the research suggests.
Children should get at least an hour of exercise a day.
Many of the 400 children enrolled in the study did less than this as they got older.
The experts from Glasgow and Newcastle tracked the activity levels of the children over eight years using monitors worn for a week at a time.
The amount of exercise the children did was measured at age seven and then again at age nine, 12 and 15.
