input: Write a python function to calculate the average age of users from a JSON input.
  assert calculate_average_age('[{"name": "Alice", "age": 30}, {"name": "Bob", "age": 20}, {"name": "Charlie", "age": 25}]')==25.0
  assert calculate_average_age('[{"name": "Dave", "age": 40}, {"name": "Eva", "age": 35}]')==37.5
  assert calculate_average_age('[{"name": "Frank", "age": 50}]')==50.0
output: import json
  
  def calculate_average_age(json_input):
      users = json.loads(json_input)
      total_age = sum(user["age"] for user in users)
      return total_age / len(users)