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Donn Trenner, 91, estimates that two-thirds of his friends are dead.

“That’s a hard one for me,” he said. “I’ve lost a lot of people.”

As we age, we not only lose friends, but face the daunting task of making new friends.

And it turns out that friendship plays a critical role in health and well-being, according to recent findings from the Stanford Center on Longevity Sightlines Project. Socially isolated elders face health risks comparable to those of smokers, and their mortality risk is twice that of obese individuals, according to the study.

Being disengaged from friends, family members and neighbors can make building new friendships even harder, said Laura Carstensen, director of the Stanford Center on Longevity. “If we’re disengaged, it’s going to be harder to make new friends,” she said.

“Friendship is not to be taken for granted,” said Trenner, a pianist. “You have to invest in friendship.”

Even in your 90s, the notion of being a sole survivor can seem surprising.

Perhaps that’s why 91-year-old Lucille Simmons of Lakeland, Fla., halts midsentence as she traces the losses of friends and family members. She has not only lost her two closest friends, but a granddaughter, a daughter and her husband of 68 years. Although her husband came from a family of 13 children, his siblings have mostly all vanished.

“There’s only one living sibling — and I’m having dinner with him tonight,” said Simmons.

Five years ago, Simmons left her native Ohio to move in with her son and his wife in a gated 55-and-older community near Orlando. She had to learn how to make friends all over again. Luckily, she was up to the task.

Simmons takes classes and plays games at her community center. She also putters around on a golf cart (which she won in a raffle) and invites folks to ride along with her.

For his part, Trenner doesn’t need a golf cart. His personal formula for making friends is music, laughter and staying active.

Simmons has her own formula. It’s a roughly 50-50 split of spending quality time with relatives and friends.

Experts say they’re both doing the right thing by retaining existing connections, remaining open to new friendships and constantly creating new ways to seek them out.

At any age, genuine friendships require repeated contact, said Andrea Bonior, author of “The Friendship Fix: The Complete Guide to Choosing, Losing and Keeping Up With Your Friends.” That’s why she advises that older folks join groups that meet on a regular basis, such as exercise classes or knitting or book clubs.

She also suggests that seniors get involved in “altruistic behavior” like volunteering in a soup kitchen or animal shelter or tutoring English as a second language.

“Friendships don’t happen in a vacuum,” she said. “You don’t meet someone at Starbucks and suddenly become best friends.”

Bonior recommends that seniors embrace social media, which can help older people strengthen their relationships with nieces, nephews and even grandchildren, said Alan Wolfelt, an author, educator and founder of the Center for Loss and Life Transition.

“It’s important to create support systems that don’t isolate you with your own generation.”

Who your friends are isn’t as important to Carstensen as the fact that you have them.

“It’s the quality of the relationship that matters most,” she said.