BURLINGTON, Iowa — There are plenty of reasons to wonder whether a man who will be 75 on Inauguration Day is too old to be president. But watching Bernie Sanders speak in the livestock arena of the Des Moines County Fairgrounds on a hot night in early September suggests those doubts might not apply to him.

The temperature at sunset was around 90, and the humidity higher, when Sanders began to speak. Spots of sweat soon appeared on his shirt. Then large patches of sweat. Then the entire shirt was drenched. Thirty minutes passed, then 45, then an hour, and Sanders was still going strong — if anything, getting stronger. He knocked off after an hour and six minutes. It was his last event of a long day, after all, but he sounded as if he could go for hours more.