What a nuclear attack didn't take from them, old age will.

Seventy years have passed since the United States shocked the world by dropping atomic bombs on the Japanese cities of Hiroshima and Nagasaki. As nuclear powers gather this week to discuss a landmark disarmament treaty, the now-fragile survivors warn this may be their last chance to use their personal horror to hurry that work along.

The Nuclear Nonproliferation Treaty, or NPT, review conference happens just every five years. The average survivor, called hibakusha, is about 80 years old.

There are signs they are fading. Sunday's thousand-strong rally in Manhattan against nuclear weapons was led by a trio of women in wheelchairs, slowly making their way along the mile-and-a-half route.

One of the most famous survivors, 86-year-old Sumitero Taniguchi, could not join them. Frail and silent, he sat in a wheelchair and watched the march set off up the avenue. Japanese media surrounded the Nagasaki survivor with cameras, leaning in for a better view.

"I was shocked when I saw him," said 83-year-old Hiroshima survivor Setsuko Thurlow, who completed the march in a wheelchair. "He was so much thinner."

Thurlow was 13 when Hiroshima was bombed and 140,000 people were killed. "People at a distance saw the mushroom cloud and heard a thunderous roar. But I did not see the cloud because I was in it," she says when she tells her story. She climbed from flaming ruins and spent hours giving water to a field full of people dying.

She has spoken about her experience, she said, thousands of times around the world. "My health? I've been blessed," she said after Sunday's march. But friends say she lives with pain every day.

Japan is famous for having the world's oldest population, but supporters of the hibakusha warn that the health effects of a nuclear attack likely will shorten their lives.

Taniguchi was 16 years old and riding his bicycle about a mile away from the epicenter when Nagasaki was bombed. Seventy thousand people were killed. His back was so badly burned that he spent much of the next three and a half years lying on his stomach. His friends say he still has open wounds and cannot sweat.

"Nuclear arms are weapons of the devil, which will not allow humans to live nor die as humans," he told an audience at a disarmament gathering in New York over the weekend.

Later, waiting for an elevator to go up one floor, a chain of origami peace cranes around his neck, Taniguchi declined an interview request. Too tired, his friend Hiroko Kumagai explained. "I am very worried about his health condition," she said quietly.

"It's a miracle he's alive. And he's a super heavy smoker," said Kathleen Sullivan. The program director for a project called Hibakusha Stories said this year will be the last to bring survivors to speak at New York City schools, "because frankly, it's concerning bringing so many elderly people together."

Sullivan teared up for a moment over the recent death of a good friend, a survivor who was in his late 80s. She worries about how to carry stories forward as a unique generation passes away.

In the past few years, trainings have begun in Hiroshima and Nagasaki so that people can share the experiences of specific survivors, she said. But the personal impact will be missing.

The U.N. secretary-general, Ban Ki-moon, has made a special point to thank the hibakusha. "I defy anyone to look into the eyes of these courageous and resilient individuals and say you know better what nuclear weapons bring," he said in a statement as the nuclear conference opened.

About three dozen hibakusha came to New York for the nuclear conference, but it's the last time a significant number will be able to make the trip, said Joseph Gerson, disarmament coordinator for the American Friends Service Committee.

Pain and shame around radiation exposure at Hiroshima and Nagasaki lifted slowly in Japan, inhibiting public discussion. Hibiki Ouchi's grandfather was a Hiroshima survivor who later died of skin cancer, but no one told her his story until a few years ago, when her mother got breast cancer and worried that it was because of the bomb.

By then, Ouichi was already working with the Japan Council Against Atomic and Hydrogen Bombs.

"I never met my grandfather, but maybe he made me do this," she said. "Whenever I hear a hibakusha testimony, I feel they are him."

Hopes now rest on the youngest survivors like Toshiki Fujimori, 71. "We younger hibakusha are trying to inherit the experience," he said.

But he remembers nothing of his own. When Hiroshima was attacked, he was 1 year old.