Karl Pillemer, a Ph.D. gerontologist at Cornell University, has spent the last several years interviewing hundreds of older Americans to systematically collect their practical wisdom.

His first book, 30 Lessons for Living, synthesized advice from over 1,000 elders on topics like happiness, work, and health.

In January, Pillemer followed up with 30 Lessons for Loving, which features lessons from over 700 older Americans with 25,000 collective years of marriage experience. One couple he profiles was married for 76 years. Another interviewee describes divorcing her husband, then remarrying him 64 years later.

Among other questions, he asked these oldest Americans what people tend to regret at their age, and what they would advise younger people to do to avoid regrets.

One message was expressed in different forms but repeated again and again, with great emphasis:

In an interview with The Huffington Post, Pillemer expressed surprise at how many elders said that their greatest regret was not traveling enough.

Many believed it wasn't worth the costs; others put it off after having children; and still others decided to wait until retirement. An 81-year-old named Jack, whose wife Lynne died of cancer after they retired, shared his heart-breaking experience of realizing he had waited too long.

"We always thought we'd do a lot of traveling when we retired, you know? But then Lynne passed away, and it was too late. I went on a couple of trips and I guess they were okay, but it's less fun going alone. I took a bus through the Canadian Rockies, and I actually turned once to talk to her -- I was sitting in a seat by myself and it was beautiful, and I wanted to tell Lynne, 'Look at that light, the color, that light.' But of course she wasn't there. And I just want to share things with her when I travel, but we waited too long."

Pillemer, summarizing the collective advice of these elders, said, "Travel is so rewarding that it should take precedence over other things younger people spend money on."

Travel is especially valuable to people in the early stages of a relationship, Pillemer noted.

"That period when you are meeting people from all over the world and thrown onto your own resources, detached from family and other pressures -- it's a truly life-changing experience," he said, referencing his own travel in Germany after college with his spouse-to-be. "To go through that with someone, where you're dealing with those kinds of challenges together, it's sort of a signature way to start a relationship and really be reliant on one another."

Related: He Asked 1500+ Elders For Advice On Living And Loving. Here's What They Told Him. Transcription services by Tigerfish; now offering transcripts in two-hours guaranteed. Sophia is a project to collect life lessons from fascinating people. Learn more or sign up to receive lessons for living directly via Facebook or our email newsletter.

He continued: "That's one thing that the elders would recommend: travel's great at any age, but particularly important when you're young, and if it can be part of a relationship that you're building, it's an experience not to be missed."

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