input: Write a function to classify users based on their age into "young", "middle-aged", or "elderly".
  assert classify_ages([25, 35, 65])==["This user is young.", "This user is middle-aged.", "This user is elderly."]
  assert classify_ages([20, 45, 55])==["This user is young.", "This user is middle-aged.", "This user is elderly."]
  assert classify_ages([29, 30, 50])==["This user is young.", "This user is young.", "This user is elderly."]
output: def classify_ages(ages):
      classifications = []
      for age in ages:
          if age < 30:
              classifications.append("This user is young.")
          elif 30 <= age < 50:
              classifications.append("This user is middle-aged.")
          else:
              classifications.append("This user is elderly.")
      return classifications