Perl 6 Private Methods

human.p6

class Human {
  has $.name;

  has $!age;
  method age {
    ($!age * 0.8).round;
  }

  method new (:$name, :$age) { self.bless(:$name, :$age); }
}

my $john = Human.new(name => 'John', age => 35);
say $john;      #=> Human.new(name => "John")
say $john.name; #=> John
say $john.age;  #=> 0
                #!> Use of uninitialized value of type Any in numeric context
                #!> in method age at /home/jrusso/exercism/human.p6:5