President Biden underwent the third physical exam of his presidency on Wednesday amid concerns over his age as he campaigns for a second term.Mr. Biden, 81, traveled to Walter Reed National Military Medical Center in Bethesda, Md., for his annual physical, according to the White House, and spent about two and a half hours inside Walter Reed. The White House is expected to release a summary of Mr. Biden’s physical later on Wednesday.“They think I look too young,” Mr. Biden joked as he answered questions from reporters at a round table with law enforcement officials at the White House…