A study published on Wednesday in The Journal of the American Medical Association showed that 10-year-old American children become lazy once they grow up. 90% of the 9-year-old children get more exercise than a 15-year-old.

The government recommends an hour of moderate-to-vigorous exercise a day for the children aged 10 to 15, which could be fulfilled by walking, cycling, jogging or swimming. But less than a third accomplishes these requirements.

This could be a real danger for the children’s health throughout their lives and when they reach adulthood. So much inactivity creates bigger risks for health problems like heart disease, obesity, diabetes and high blood pressure.

A week ago, a pediatricians group recommended that more children have their cholesterol checked and that the ones younger than 8 should be given cholesterol-lowering drugs.

The study, which was part of the continuing Study of Early child Care and Youth Development, tracked more than 1,000 U.S. children at various ages, from 2000 until 2006. The doctors used special machines to record the children activity and the results showed that levels of moderate-to-vigorous activity fell from three hours a day at age 9 to less than an hour at age 15.

One of the doctors said that schools are guilty for this decline, as their programs have dropped recess and gym class and let the children get more interested in video games and computers. The study advises parents to balance their children’s time, by teaching them that walking the dog or shooting some hoops matches perfectly to a pause from the computer.