Why the change? Some experts blame it on overprotective parents.
"The culture's a bit different. It's a little bit scarier to let them go for four hours and not know exactly where they are," said parent Catherine Hegley.
But studies show American kids are safer now than they've been at any time since 1975. Violent victimization of kids has dropped more than 38-percent.
As outdoor play among children has dropped 50-percent over the last 20 years, health problems are on the rise. Child obesity rates have more than doubled -- an epidemic that could cost the health system $100 billion each year.
Studies have also linked time outdoors to better grades, concentration, self-esteem and imagination, and it's even been shown to reduce symptoms of ADHD. Janet Milkman works to link kids with nature. She says kids are healthier, happier and smarter after time outdoors.
"Nature is a way for kids to learn independently and to explore. It helps them to be out of the pressure of an urban environment -- a structured environment," said Milkman.
At the Schuylkill Center for environmental education in Philadelphia, nature is part of the curriculum.
Just a short hike can increase a sense of wonder, and decrease the chance kids grow up disconnected to the outdoor world around them.
Experts say schools across the U.S. are tripling their spending on technology and slashing art and environmental programs. The National Wildlife Federation recommends parents set aside an hour outdoors every day.