My grandpa started walking five miles a day when he was 60.

This joke gets its chuckle from a classic comedic misdirection (Review), blending a touch of irony with outright absurdity. The setup builds an expectation that grandpa's daily five-mile strolls are a testament to his vibrant health and dedication to fitness, a truly admirable feat for someone starting at 60. We're primed to hear about his amazing vitality at 90.

However, the punchline hilariously yanks the rug out from under that expectation, revealing that his consistent exercise has led not to glowing health within the family's view, but to him being completely lost. It's a playful twist on the well-intentioned advice often given to seniors about staying active and keeping fit to maintain independence and a good quality of life. The humor comes from taking that good advice to such an extreme, and then presenting a completely unexpected and ridiculous consequence.

The joke pokes fun at the idea that more of a good thing is always better, especially when it comes to exercise. Instead of a heartwarming tale of a spry nonagenarian, we get the image of a super-fit, but utterly directionless, elder, leaving his family scratching their heads. It’s a silly, lighthearted jab at life's unpredictable outcomes, even when following the best intentions.