user.rb

class User
  attr_accessor :age
  def initialize
    @age = 0
  end
  
  def birthday
    puts 'happy birthday!'
    birthday_act
    @age += 1
  end
  
  private
  def birthday_act
  end
end

class Boy < User
  private
  def birthday_act
    puts 'some process...'
  end
end