"I try to live the right kind of life," he said. "I try to live the truth, I love everybody and I want everybody to love me."

Some experts say a positive attitude like Harris's is a factor in longevity. Others point to a connection with how fast a person walks.

Doctors say walking speed is a reflection of how well many of the body's systems are functioning.

Whatever the reasons, Dr. Neil Buckholtz, at the US National Institute on Aging, says governments the world over need to prepare.

"The population, not only in the United States, but worldwide, of older people is increasing," he said. "And actually, the fastest-growing group of people in the United States are those people over 85."

According to the World Health Organization, in the next five years, the number of 65-year-olds and greater will surpass the number of children under the age of five. That's why the focus of this year's World Health Day is on healthy aging.

Dr. Enrique Vega specializes in elder care at the Pan American Health Organization, which is part of the WHO.

"There is not any relation between age and pain or between age and disease," said Vega.

Dr. Vega says pain and disease can be treated at any age. He says healthy aging involves staying socially and physically active. The players on this softball team are doing both, Many are in their late 60's and mid-70's. Shirly Thompson is 75. 

"I have arthritis in my fingers," she said. "See the knots on my hand. I think that is one good thing about playing ball. It does help your arthritis from stiffening up on you." 

Dr. Vega says governments need to help the elderly learn to care for themselves.

"It's a complex process to manage your chronic disease and to avoid disability," he said. "And therefore people need to prepare for that."

Dr. Vega says countries also need to develop targeted health programs for the elderly to minimize costs. 

"They are not prepared now. And this is one of our main weaknesses," he said.

And that's all the more reason the Pan American Health Organization, as well as the entire WHO, is drawing attention to healthy, active aging on this year's World Health Day.