Most experts believe that physical activity declines as children reach adolescence, but a new study has found that the decline begins as early as age 6.

Researchers measured physical activity at ages 6, 8 and 11 in 600 European children wearing arm band activity trackers. After adjusting for body mass index, the scientists found that average time spent in physical activity declined by about 75 minutes a day by the time the children were 11. Light physical activity declined by 45 minutes a day, and moderate to vigorous activity, which remained steady until age 8, declined an average of 31 minutes a day by age 11.

The study, in Pediatrics, found that at age 11, the children were spending an average of almost two more hours a day in sedentary behavior — essentially, sitting still — than they were at 6. Only 63 percent of 11-year-olds were getting the recommended 60 minutes a day of moderate to vigorous exercise, compared with 82 percent of 6-year-olds.

Boys got more moderate to vigorous exercise than girls, and girls more light activity than boys, but the overall decline in exercise was sharper in boys.

“The only major influencing factors were sex, country and body size,” the authors write. “Interventions to prevent inactivity might need to focus more on younger children.”