difference between using @ over self

dog_.rb

class Dog
  def age
    @age
  end

  def initialize(age)
    @age = age
  end
end

d = Dog.new(8)
puts d.age

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class Dog
	attr_reader :age
	def initialize(age)
		@age = age
	end
end

d = Dog.new(8)
puts d.age

dog_self.rb

class Dog
  def age
    @age
  end

  def age= age
    @age = age
  end
  def initialize(age)
    self.age = age
  end
end

d = Dog.new(8)
puts d.age