(CNN) Frank Dewhurst was walking in his neighborhood when he saw the sign in his neighbor's yard.

"I am type O and I need a kidney transplant. Please help me."

The sign gave him pause. He was type O+.

Linda Nall, the neighbor who had put it up, had been struggling with lupus since 1986, and the disease started to attack her kidneys in 2001. She had trouble getting out of her car, which Dewhurst had witnessed, and had to maintain a strict diet. He'd also seen the signs she'd put on the back of her car, as well as her social media posts.

This time, he popped by for a visit.

That was late last year. Now Nall, 72, has a new kidney -- Dewhurst's.

And Dewhurst, at age 84, is now the oldest living kidney donor in the United States.

The decision to go through with it

Dewhurst is involved in the homeowner's association of their Austin, Texas, neighborhood, so when he knocked on Nall's door, she thought he was going to ask her to take down the signs, Dewhurst said.

"(She said) something like, 'That's the last thing we were expecting from you,'" he told CNN.

Linda Nall, 72, put signs in her yard and on her car searching for a kidney donor.

Dewhurst said had read about kidney donors before, so he knew it was possible to live with one kidney.

One thing stayed on his mind, though: an article from the AARP magazine that he'd seen maybe a year before.

Three friends in Arizona had gone golfing together and one of them had kidney failure, Dewhurst said. Both friends got tested, and one was a match and able to donate their kidney.

Dewhurst recalled thinking the friends in the article had to be at least in their 60s. If they could do it, why couldn't he? After discussing his idea with his wife, who was in full support, he went for it.

They were a perfect match

More than 5,000 kidney donations happen every year, but only 200 donors in the past 24 years have been older than 70.

There are a lot of factors that prevent older people from donating. The healing process takes longer, and there's a strict physical and psychological evaluation all participants must undertake.

And kidney function declines with age, with most adults losing about 1% of functionality each year after age 40, said Dr. Hassan Ibrahim, who treated Dewhurst at Houston Methodist Hospital.

Dr. Hassan Ibrahim evaluated Dewhurst before the operation at Houston Methodist.

So assuming someone starts at 100% functionality, by age 80 their kidney may only be at 60% functionality.

But fewer than half of older adults don't have a significant decline in kidney function, and that's the category Dewhurst fell in.

This decline in kidney function becomes a problem when an older person donates a kidney to someone significantly younger. An 84-year-old donating to a 72-year-old was almost perfect.

"When he approached us, I actually did not put him through the physical evaluation process," Ibrahim said. Instead, he met with Dewhurst for an hour to discuss the issues. Houston Methodist had taken kidneys from other older patients before, one 80-year-old and one 79-year-old, but never someone as old as Dewhurst.

Ibrahim said he was impressed. Dewhurst was active for his age and not on any medications, making him an ideal candidate.

Now, she has many more years left

Dewhurst is now the country's oldest living kidney donor, the United Network for Organ Sharing says. He's 85 days older than the person who held the record before him.

But what if Dewhurst hadn't stepped in?

Nall was briefly on dialysis before Dewhurst donated his kidney. The average survival period for a 72-year-old on dialysis is about five years, which is also the average wait time in Texas for a kidney from an organ donor, according to Ibrahim.

A live donor transplant like the one from Dewhurst extends the average survival time to 12 years.

Dewhurst made several trips to Houston from his home in Austin -- one trip was just two full days of testing. But he says he wasn't nervous about his age or any possible problems. After all the testing Houston Methodist put him through before the procedure, he told CNN he was confident he was in good hands.

"I just hope it motivates others that are healthy, no matter what age, to donate," he said. "And hopefully somebody 85, 86, will donate. No big deal. It's a number."

Both Dewhurst and Nall are now doing well.

"I am going to make the most of Frank's generous gift and live life to the fullest," Nall said in a news release from Houston Methodist. "I cannot wait."