ATLANTA >> Forget turning 90. The birthday Rosalynn Carter really wants to talk about is her first one in the White House.

“I turned 50, and I thought that was really bad,” Carter, 89, recently recalled with a soft chuckle. If she could go back in time now and talk to that younger version of herself, she said, she’d be able to reassure her: “You can have a full life after 50.”

You can say that again. And again, when it comes to Carter, the once shy, small-town girl from Plains, Georgia, who became a groundbreaking first lady — and then went on to accomplish still greater things after her husband lost his bid for a second term as president in 1980.

Just days shy of her 90th birthday on Aug. 18, Carter sat down to speak about the milestone and her remarkable life. Even before she went to Washington, she’d served as first lady of Georgia from 1971 to 1975, becoming a passionate, effective voice for improving mental health care, immunizing children against preventable diseases and other important issues.

After she came back to Georgia, this mother of four and grandmother of 12 co-founded the Carter Center, which fights disease and promotes human rights around the world; helped establish the Rosalynn Carter Institute for Caregiving (RCI) long before the myriad issues faced by professional and family caregivers was on many people’s radar; and along with her husband became pretty much the world’s best-known Habitat for Humanity volunteer, thanks to their annual Carter Work Project, which has built more than 4,000 houses around the world.

True to form for someone so devoted to public service, she hoped to keep this birthday interview — the only one she was giving — focused more on the important issues she’s advocated for tirelessly for decades, in particular, mental health and caregiving.

“I’ve had a great life,” Carter said in her office at the Carter Center in Atlanta, where large windows afford a view of the grounds and the shelves are filled with books and photos. “I’ve watched my family grow, I’ve traveled around the world and I’ve had a chance to contribute some, I think.”

“Some” doesn’t begin to do her justice. In October, the RCI — whose roots lie in “the most emotional meeting I’d ever seen” of local caregivers, many of them tending to sick or elderly family members, discussing burnout and other concerns — will host its 30th Anniversary Summit as a nationally recognized research, education and advocacy institute based at Georgia Southwestern State University in Americus.

MEANWHILE, Carter refused to be deterred during her early days as a mental health advocate, when the stigma surrounding mental illness made it hard to get people to come to meetings, “even with me being the governor’s wife.”

Under her more than three decades of leadership, the Carter Center’s Mental Health Program has become a major force in the field, hosting an annual symposium of national mental health leaders to form policy and creating a journalism fellowship program to encourage accurate reporting about mental health issues, among other things.

Not that she’s ready to take her foot off the gas just yet.

“It’s just great progress, but we still have a long way to go,” Carter said. “We worked all these years to try to overcome the stigma and it still keeps so many people from getting help. Which is so sad because today they can recover and the overwhelming majority can become contributing citizens in the community.”

Still, what’s a chat about turning 90 without an unscheduled appearance by an obvious expert on the subject? At one point during the 25-minute interview, Jimmy Carter popped in to say hello and to disclose that he and his wife of 71 years had been discussing that very topic the night before.

“It’s no problem,” Carter, who’ll turn 93 himself on Oct. 1, said genially about hitting the big nine-oh. The former president appeared fit following the dehydration scare that briefly hospitalized him in Canada during a Habitat for Humanity building blitz in mid-July (he continues getting regular scans to ensure that his brain cancer, which went into remission months after he began receiving treatment for it in August 2015, hasn’t returned).

Of the girl-next-door he quite literally married (their two families lived beside each other in Plains when she was born), he added, “I’m just thankful that she’s gotten that far.”

She’d already received an early birthday present from him, Mrs. Carter confided after he’d popped back out of her office again: a robotic vacuum cleaner, which her grin clearly indicated she’s getting a kick out of deploying around the ranch house in Plains where they’ve lived since 1961.

BEYOND THAT, asked how she’ll celebrate on Aug. 18, she said, “I haven’t thought of anything, except that I’m going to be at home, and that’s exciting.”

There will be a public party of sorts in Plains the next day: A concert, “Summer Jazz in Celebration of Rosalynn Carter’s Birthday,” will be held Saturday evening at the old Plains High School, preceded by a cocktail party downtown. Ticket sales will benefit two local nonprofit organizations, which friends and relatives of the former first lady say is pretty much the only way she’d ever agree to them making such a fuss over her.

Yet nothing may top what she describes as her most memorable birthday.

“My staff gave me a ride in the Goodyear Blimp,” Carter recalled near the end of the interview. “They had to draw straws for who would go with me because we couldn’t get everybody in. I had a great birthday!”

That was her 51st birthday, her second one in the White House.

Turns out she was right all along. You can have a full life after 50.

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