He was working as a teacher and had heard that young men like him were being drafted into the military.
He made a quick decision to get out there and then.
For nearly seven decades since that fateful day, Ahn has only been able to imagine what life has been like for his family on the other side of the heavily militarized border that separates North from South Korea.
"Just by looking in that direction, I can remember their faces," he tells CNN.
Ahn has been applying for a family reunion since the program started in the 1980s.
Finally, at the age of 85, he's getting a chance to be reunited with his sisters -- and he knows exactly what he wants to say to them.
"I would ask my younger sisters to forgive me. I left them with all the responsibilities. My heart is breaking. I abandoned them."
The reunions are a rare opportunity; this is only the twentieth that has ever been held. Less than 200 families are taking part -- 96 families from the North and 90 from the South.
According to the South Korean Unification Ministry, which handles inter-Korean affairs, more than 130,000 South Koreans have applied since 1988.
The majority, like Ahn, are in their 80s and 90s and are running out of time. About 64,000 of those applicants have already died.
However, the reunions have been used as a political tool -- canceled at times of heightened tensions between the two neighbors. The last time they were held was in February last year.
This round was agreed to as part of the deal reached during high-level talks between officials from North and South in August, after an exchange of artillery fire across the border provoked an escalation in tensions as both sides ramped up their war preparedness.
Some had speculated that North Korea might renege on its promise to go through with the reunions, as it has done in the past. But there have been relatively encouraging signs recently. For example, earlier this month, Pyongyang released New York University student, Won-moon Joo
, who had been detained for illegally entering the country in April.
State news agency, KCNA, said the release was "in line with a humanitarian measure."
North Korea has also refrained from provocative actions likely to test the patience of Seoul and its chief ally, the United States, such as a satellite launch or a nuclear test to mark the recent 70th anniversary of the foundation of the Korean Workers' Party.
Robert Kelly, Associate Professor of Political Science and Diplomacy at Pusan National University, sees all of this as concessions of sorts from Pyongyang.
"North Korea is not doing this to be nice," he says. "North Korea does not give up anything meaningful without getting something in return."
One thing Kim Jong Un's regime gets out of this, he says, is good press, and it doesn't cost North Korea anything to follow through.
"It makes North Korea look less like this goose-stepping frightening place," he explains. "It is good PR for a regime that rarely gets good PR."
But the interaction is also heavily monitored.
"There is a debate about whether North Korea sees this as a threat," says Kelly. "It's new information, it's unregulated information."
Meanwhile, Ahn knows that many topics that could be seen as controversial will be off-limits when he meets his sisters. But he feels extremely lucky to be selected anyway.
"Words cannot express how happy I am," he says. "I feel like I am meeting people who came back from the grave."
But the letter he received confirming that his application was successful also contained some bad news -- it revealed his youngest sister died in 1983.
He does not know how she died, but he's convinced it would not have happened if she had lived in South Korea.
"In this part of the world, we could have cured her illness. There are no medicines or medical facilities there. My heart really ached."
He also had two brothers -- twins. But they were never found.
The reunion with his two remaining sisters will be bittersweet. They will only have a matter of hours to spend together, to catch up on the lifetime they missed. This time Ahn will say goodbye, but he will do so knowing it will almost definitely be for the last time.