Hong Kong has disqualified a 21-year-old democracy advocate from running for the local legislature, the latest in a series of young politicians to be barred from public office over their resistance to the Chinese government’s authority over the city. The pro-democracy party Demosisto said on Saturday that election authorities had barred its candidate, Agnes Chow, from running in a March by-election, in which Hong Kong voters will decide on members of the Legislative Council. Ms. Chow said the ruling was “no less than a declaration to the city that our political rights are handicapped.”