July 15 (HealthDay News) -- By the time children reach their teens, their level of physical activity drops significantly, new research shows.

Kids who were averaging three hours of moderate to vigorous activity when they were 9 barely manage to get more than a half-hour of daily exercise by the time they reach 15, according to a study in the July 16 issue of the Journal of the American Medical Association.

"Kids' activity is decreasing dramatically between 9 and 15," said study author Dr. Philip Nader, professor emeritus of pediatrics at the University of California at San Diego in La Jolla.

Nader said the reasons for the drop are many.

"There may be competing, more interesting things to do; physical education is being done away with in some places, and so is recess; there aren't as many open spaces or parks, and being outside is one of the main things that keeps people active," he said.

Plus, children don't get the same routine daily activity that youngsters from a generation or two ago did. "Kids used to just run around and ride their bikes everywhere, and kids used to walk to school. Now, parents drive them," Nader noted.

The lack of physical activity is linked to the growing problem of childhood obesity, and most experts recommend that children should be getting at least one hour of moderate to vigorous activity each day, reports the study.

To assess how close children were coming to that goal, Nader and his colleagues followed a group of 1,032 kids beginning in 2000, when the children were 9, until 2006, when they were 15.

Half of the group was male, and almost one-quarter came from low-income families.

At 9, the children engaged in an average of three hours of moderate to vigorous physical activity each day. From that point on, daily exercise dropped an average of 38 minutes per year on weekdays and 41 minutes per year for weekend activity. At 15, teens participated in daily moderate to vigorous activity for an average of 35 minutes on the weekend and 49 minutes on weekdays.

Boys stayed more active than girls, averaging an extra 18 minutes on weekdays and 13 more minutes on the weekends. Girls dropped to less than one hour of activity daily on weekdays at about 13.1 years old, while boys didn't reach that mark until 14.7 years.

"This drop-off has always been there. Very young kids are extremely active, and as kids get older, they get into different lifestyles and different activities," said Dr. Goutham Rao, clinical director of the Weight Management and Wellness Center at Children's Hospital of Pittsburgh.

"Games like hide-and-go-seek get displaced by homework and computer time," he noted.

What's really missing today, he added, is the everyday activity. "Kids don't walk to school anymore, and that's probably the most physical activity they might get. No one uses stairs or rides bikes anymore. And, our environment isn't always suitable for those things," said Rao.

"Parents need to recognize that activity is an important part of kids' routine, just like schoolwork," said Rao.

Nader added that exercise should really be a family affair, and that parents need to model good exercise behavior.

SOURCES: Philip Nader, M.D., professor emeritus, pediatrics, University of California at San Diego, La Jolla; Goutham Rao, M.D., clinical director, Weight Management and Wellness Center, Children's Hospital of Pittsburgh; July 16, 2008, Journal of the American Medical Association