When Karl Pillemer set out to interview elderly men and women, he wasn’t entirely sure what to expect.

A gerontologist and professor at Cornell University, Pillemer had seen studies indicating that many senior citizens were happier than people who were decades younger, despite being plagued with chronic disease, various disabilities and sagging skin. He wanted to know why — or, more to the point, how.

Pillemer anticipated empty generalization. He dreaded the phrase “Live life to the fullest.”

Instead, over the past six years, as Pillemer and his research team talked to 1,500 people age 70 and older, they gathered thousands of pages of transcripts filled with practical guides to a happy life: Don’t rush into marriage. Don’t hit your kids. Don’t choose a career just because it pays well. Being old is better than you think.

Clearly, these people had something to say. “The problem is, no one’s been willing to listen,” said Pillemer. His ongoing study, which he calls “The Legacy Project,” has given a voice to the unheard — and turned some conventional thinking about aging on its head.

Book cover, “30 Lessons for Living: Tried and True Advice from the Wisest Americans” by Karl Pillemer.

When Pillemer turned 50, he recalled, his own anxiety over aging and what began to feel like “imminent death” grew. Then he had a conversation with a nearly blind, waxy-skinned, jubilant 90-year-old named June. “Young man,” June told him, “you will learn, I hope, that happiness is what you make it, where you are. . . . It’s my responsibilitity to be as happy as I can.”

That interview set him on a different path. “A lot of my research had been focused on the problems of older people, or older people as problems for society,” said Pillemer, now 57, ticking off medical-care costs, elderly abuse and depression as examples. “But I was meeting older people who seemed really happy.”

And he thought he knew a way their experience could be put to good use. There’s definitely a market for advice in this country: Americans spend more than $10 billion each year on such self-help products as books and coaching, according to Marketdata Enterprises, a Tampa-based research firm.

“Up until 100 years ago, people wouldn’t think of going to anyone other than older people for advice,” Pillener said. “But we’ve lost that instinct.”

So he created the Legacy study and compiled his research into “30 Lessons for Living: Tried and True Advice from the Wisest Americans,” which was published last year. He continues to ask more people to post “life lessons” on the project’s Web site.

He also is trying to expand connections between elderly people and those just starting out in life by developing interactive programs for students in middle school, high school and college.

“Instead of going and singing Christmas carols in a nursing home, young people can have conversations,” Pillemer said.

Several of the lessons he has heard have resounded personally with Pillener, he says, but the one that has changed his approach to life is the advice not to worry about things he can’t control.

“One woman told me that waking up in the middle of the night, panicking and staying awake ruminating about life — that’s a 35-year-old’s game,” he said with a laugh. “People in their 90s talked about finally feeling ‘free.’ ”

That’s not to say every long life has a happy ending. “The lessons aren’t all from exemplary, positive, high-functioning people,” Pillemer said. “Many of them are in bad shape. Many have regrets. But the view from 80, 90, 100 years old it that life is really short. It’s precious. And we shouldn’t waste it worrying about getting old.”

To learn more about the Legacy Project or to submit your own life lessons, go to legacyproject.human.cornell.edu .