I update it to pass below test.

Use a subslice as shallow copy doesn't work too.

  It("should not mutate source for any input", func() {
    data     := []int{1,5,87,45,8,8}
    expected := []int{1,5,87,45,8,8}
    TwoOldestAges(data)
    Expect(data).To(Equal(expected), 
      "Mutates %v", expected)
  })

Thanks, I"m pretty new in golang world :) Love your solution!

sort mutates source.

So to avoid side effects, it must be copied before use.