For more than half a century, a cheeky little sign hung behind the bar at Wintzell's Oyster House in Mobile, Alabama. It read: "Free oysters to any man 80 years old accompanied by his father." Most customers read it and laughed. Jimmy Rush read it and started planning.

"We kept asking, 'Was this sign for real?' and they said yes," Jimmy told CBC Radio's As It Happens. "We said, 'Has anybody ever done it?' And they said no."

A pact sealed in the 1970s

The Rush family first spotted the sign back in 1972, during a Mardi Gras celebration. Jimmy and his brother Carl looked at each other and looked at their father — Jim Rush Sr., already a devoted oyster man — and made a pact. They'd be back when the time was right.

The sign had been hung by Wintzell's original owner, J. Oliver Wintzell, a man fond of quirky signage who figured no 80-year-old would ever have a living father to bring along. It was a joke, essentially — a safe bet in the mid-20th century.

But nobody told Jim Rush Sr.

54 years of checking in

The decades rolled past. The OPEC embargo. The fall of the Berlin Wall. The internet. Every so often, the Rushes would call the restaurant to double-check: was the deal still on? It was. And Jim Sr. kept walking his four or five miles a day, eating his oysters, and staying stubbornly, wonderfully alive.

"I've only been sick twice in my life — once when I was five and once when I was 97," Jim Sr. said. "I don't take any medications at all."

When asked about the secret to his longevity? "I don't know. I've eaten a lot of oysters is the only thing I can say," he told NPR's Scott Simon. His son Jimmy chimed in from the background: "Jack Daniels."

The big night

Last month, on Jimmy's 80th birthday, the pair finally walked into Wintzell's — arm in arm — to claim what was theirs. They brought around 60 friends and family along for the ride.

"Most people read that sign behind the bar and laugh," the restaurant wrote on Facebook. "But tonight, Jimmy Rush walked in on his 80th birthday with his father, James Rush, 99, right beside him, and turned one of Wintzell's rarest traditions into a real-life milestone."

Current owner Bob Omainsky honoured the promise in full. Oysters on the house — as many as could be eaten. And since the sign says "80 years old," not just "on your 80th birthday," the deal holds for Jimmy's entire 80th year, so long as Dad tags along.

They're not done yet

The Rushes have already been back three times. And they have plans. Brother Carl, who turns 78 in October, has already booked his 80th birthday at the same spot. "My dad said he was planning on being there," Carl told TODAY.com, "so he didn't leave me many options."

Jim Sr. — who served in World War II and Korea, worked at the post office for 30 years, and has been eating oysters since he was a boy — turns 100 this July. His daughter Dorothy, who's turning 70, is keeping her fingers crossed too. "You'll only be what — 110?" she teased her father.

Nobody in the Rush family seems to be betting against him.