There is some evidence that shows that younger children, presumably ones who aren't old enough to sit on their duffs and surf online for hours a day, are more fit, and less fat than older children who do use the computer for their primary source of relaxation.
A study published in the Journal of the American Medical Association (JAMA), in July 2008 shows that exercise levels drop off as children get older. In the study, which lasted from 2000-2006, researchers recorded the movements of more than 1,000 kids using an accelerometer, which the children wore for one week at ages 9, 11, 12, and 15. Their finding are, in a word, frightening.
The average 9 year old got about 3 hours of activity a day during the week and on weekends, and they weren't doing to badly at age 11. But by age 15, about 30% were getting the minimum recommended amount of exercise during the week, and only 17% got that much on weekends.
I know that teenagers' activities tend to slow down somewhat once they leave the rough and tumble puppy years of childhood and start sleeping until noon, but they used to have activities that helped their bodies move, even if they weren't involved in sports, such as walking to school, or to friends' houses, mowing the grass, raking leaves, and even cleaning house. Lifestyle changes have all but removed that from the list of things teenagers do -- more teens have cars than they used to, and more parents pay for lawn services than in the past, and a surprising number of kids don't even have household chores.
We also have the addition of technology to woo them away from a day tossing footballs at the lake, or taking a good walk with a friend: text messaging, cable television, movies and DVDs, online social networking, and instant messaging.
Because kids are rarely unable to think of how present behaviors will affect them in the future, (smoking, jumping off balconies into pools, drinking and driving, etc.) it may be up to you to manage their physical activities. Just like we feed them vegetables, even though the don't really want them, we have to keep them moving, even though they would rather stay plugged into something at home.
If your children plays sports -- fantastic! Make sure they stay signed up with a team. Help them to explore new sports. A rule in our house is one sport a semester -- a spring sport, such as flag football or track, and a fall sport such as Cross Country or Soccer.
But not all children are team sports kids. If you have one of those, you could try karate or dance, or even cycling. Get two bikes --one for you and one for your child, and ride together.
If none of these options will work for your family consider Frisbee, hiking, racquetball kayaking, and seasonal sports such as skiing or snowshoeing.
Make fitness a part of your family life, and your children -- even the most reluctant of teens-- will probably warm up to a family fitness activity if you keep it light, good-natured and free of criticism.