How to cast a large std_logic_vector to an array of smaller ones?

Use a function containing a loop that maps the input std_logic_vector into your census_line type, i.e.,

function to_census_line (slv : in std_logic_vector) returns census_line is
    variable result : census_line;
begin
    for i in 0 to 15 loop
        result(i) <= slv(8*i+7 downto i);
    end loop;
    return result;
end function;