By Gabe Gutierrez, NBC News correspondent 

ESTES PARK, Colo. -- At Rocky Mountain National Park in Colorado, rangers are seeing more than green this spring. They're also noticing a little more gray.

The average visitor to national parks is getting older.

Cyclist John O'Malley, 61, of Summit County, Colo., has enjoyed the trails for almost half a century.

"You do get close to nature," Malley said.

But apparently, not everyone shares that fondness anymore.

Back in 1996, at Death Valley National Park, almost a third of visitors were in their 20s. But in the last few years, that number has dropped to just 11 percent at Yosemite and six percent at Yellowstone, according to a University of Idaho analysis of Park Service attendance figures.

At Rocky Mountain National Park, the average age of visitors has risen to 46.  

"Right now, we see a lot of youth not coming to the parks," said Larry Frederick, a park ranger for more than 15 years who has noticed the changing demographics. "I think there a lot of distractions right now for young people."

Frederick said the average age of visitors used to be late 20s and early 30s.

Overall attendance at national parks has dropped only slightly in the last two years. But with fewer young visitors, some conservationists worry about what could happen in the decades to come.

"If we do not do a better job of inviting young people to the national parks and providing the funding to be able to do that the parks will become less relevant," said Tom Kiernan, president of  the National Parks Conservation Association.

So the Park Service is mounting a campaign to attract children and young adults -- the Connecting People and Parks program. On a recent Saturday this spring, dozens of kids toured a park outside Washington, D.C. 

"They get excited [and] they discover things," said Jon Jarvis, the director of the National Park Service. "For them to know that not only they can come back, but they own this place, this is their park."

Slideshow: America's national parks

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Nearly 400 national parks can be found all across America, and feature breathtaking vistas, rock formations millions of years old, and more.

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Back in Colorado, the Schafer family from Cleveland, Ohio, is bucking the trend. Three generations chose to enjoy their family vacation this year at a national park. While they are not part of the Park Service's recent outreach program, they fully support it.

"It's sad to see that the next generation will forget this," Jamie Schafer said, as she looked across a stretch of mountaintops.

She and her husband drove their kids and grandkids all the way from Ohio. Their goal: to leave their family's computers behind and nurture their love of nature for a lifetime.

"You can't capture it on a picture," her 12-year-old son, Tobin, said. "You have to be there to see it."