URBANDALE, Iowa — Older Republicans say they aren’t opposed to Nikki Haley’s call for a new generation of politicians leading their party. But when she calls for mental competency tests for politicians over the age of 75, some of them start to feel insulted. The 51-year-old Republican presidential contender has made evaluating the mental acuity of elder statesmen a key part of her opening campaign pitch. It’s one of the only ways Haley has sought to explicitly differentiate herself from former President Donald Trump, who is 76. And it’s one of the cudgels she’s deployed against 80-year-old President Joe Biden....