Jimmy Carter's grandson pays tribute as he reaches milestone birthday, 8 months after entering hospice

Nearly eight months after first announcing he would receive hospice care, Jimmy Carter -- the oldest living former president in history -- turned 99 years old on Sunday and his grandson Jason Carter is marking the occasion by celebrating his legacy.

"I think his time in the private sector and the work that he did at the Carter Center, really spending his life among the most marginalized and poorest people in the world and believing in their power to change their own lives and in the power of human rights for them and democracy for them, will be the biggest part of his legacy," the younger Carter, a former state senator and gubernatorial candidate, told ABC News.

The Carter Center, founded by Jimmy Carter and his wife, Rosalynn Carter, in 1982, spreads the belief that "human rights apply to everyone" -- a mission the Carters see as "truly their life's work," according to Jason Carter.

"In the poorest places in the world, in the tiniest villages in the world, people have the power to change their own lives," he said.

Jason Carter noted that because his grandfather entered hospice care in mid-February, the family did not believe that he would live to mark this new milestone.

"We didn't know and didn't believe at the time that we were gonna get to this 99th birthday," he said.

Asked how the former president has lived such a long life, his grandson offered a heartwarming theory.

"My grandmother," he replied, speaking of their historic love for one another -- as the longest married presidential couple in U.S. history.

"They're doing remarkably well. They're coming to the end, of course, at this time in their lives, but they are at peace, they're together, they are at home, they're in love, and you don't get much more than that and they don't expect more," he added.

Jimmy Carter made a rare public appearance at the Plains Peanut Festival in his home state of Georgia last week.

The whole family, including grandchildren and great-grandchildren, plans to celebrate with Carter down in Plains.

Those who wish to mark the former president's birthday can celebrate by going to the Carter Center's website or using the social media hashtag #jimmycarter99and to give a tribute or photo. The Carter Center has gathered thousands of tributes so far. Submissions will be collected as gifts and presented to Carter.