At a growing number of health clubs, gyms and YMCAs across the country, the treadmills and barbells aren't just for adults anymore. While experts debate whether it's a good idea, more teens and "tweens," or preteens, are hitting the workout circuit to get in shape. The trend also is driven by parents worried about childhood obesity while schools cut physical education classes. Not to mention health clubs, long considered adult-only zones that often barred youngsters, which see the potential for a new moneymaking market.

The problem is that kids are not getting enough exercise, and the health club industry is trying to respond. It is becoming very popular for health clubs to open up to children. Historically, teens and tweens have been doing things like swimming or playing basketball. You are going to see more and more specialized workouts for kids.

Whether youngsters, particularly pre-teens and early teens, should be doing individual training is still a matter of debate. While the National Strength and Conditioning Association and the American College of Sports Medicine say supervised, moderate workouts are fine, some people worry that developing bodies can be damaged by weight or cardio workouts.

"It's a myth that started 20 or 30 years ago and it's absolutely false," said Avery Faigenbaum, a professor of exercise science at the College of New Jersey, in Ewing, N.J. "Kids really need places to become physically active," he said. "In this age of budget cuts in schools, it's not happening there anymore. Recreation centers tend to be more sports-oriented, so the kids who aren't good at sports don't go. There is a niche that health clubs could fill." However, young people must be supervised, especially with weights. "I am in no way saying that an 8-year-old can come home from school and go into his basement and do weight training," Faigenbaum said. "They absolutely have to be supervised. Health clubs should either offer supervised classes or nothing at all."

Young people generally don't train hard enough to damage their bodies. But people under 16 shouldn't do serious training because of the risk of injury and because they quickly become bored. If they start to believe that working out is regimented, boring and a punishment of some kind, that lessens the chances of them continuing to work out in their adult years.

Despite the national trend, not every fitness club is kid-friendly; some are almost anti-family. Many people don't want children in their health clubs. As the infusion of family fitness continues, an adult will have to decide if it's going to bother him to have a 14-year-old working out beside him. You might have to ask, 'Will there be kids in my class?' And if you don't want that, look elsewhere.