CHICAGO — President Joe Biden may be old, but former President Donald Trump is a joke — and the butt of one — at the Democratic National Convention. Many senior Democratic officials gathered here in the Windy City say they aren’t worried that Trump and GOP officials keep trying to paint a host of top Democrats — Vice President Kamala Harris, congressional Democrats and Biden’s staff, along with “the media” — as complicit in the “cover-up” of what they argue is a mentally incapacitated commander-in-chief. ALSO READ: Joe Biden gave his best to us Want more breaking political news? Click for the …