Answer: That he'd been struck by lightning seven times
Mr. Daws had been struck by lightning seven times - once while repairing a leak on a roof, once while crossing the road to get his mail, once in a field while tending to his cows, once while sitting in his truck just minding his own business, once while walking his dog down the road, and on two other occasions. Although the running "lightning" gag was primarily utilized for comedic effect in the film, it was also the source of an important life lesson to Benjamin. Despite all the travails of old age that Mr. Daws was burdened with, he had come to regard being struck by lightning in a positive light. As he explained to Benjamin in the film, "God keeps reminding me I'm lucky to be alive".
The other answers were references to other characters in the movie. Elizabeth Abbott, the woman Benjamin met in Murmansk, was the oldest woman to swim the English Channel. Ngunda Oti, the pygmy who befriended Benjamin, had been married five times and had previously lived in a monkey house in Philadelphia.