How does the amount of physical activity that American children get on weekdays change between the ages of 9 and 15? It declines from about 10 hours a day to about 5 hours a day. It declines from about 3 hours a day to less than an hour a day. It increases steadily from about 4 hours a day to about 6 hours a day. It declines from about 1 hour a day to about 10 minutes a day