input: Write a python function to simulate the employee retrieval API endpoint.
  assert get_employee(1) == {'employee': {'id': 1, 'name': 'John Doe', 'age': 30}}
  assert get_employee(2) == {'employee': {'id': 2, 'name': 'Jane Doe', 'age': 25}}
  assert get_employee(3) == {'message': 'Employee not found'}
output: def get_employee(employee_id):
      employees = {
          1: {'id': 1, 'name': 'John Doe', 'age': 30},
          2: {'id': 2, 'name': 'Jane Doe', 'age': 25},
      }
      if employee_id in employees:
          return {'employee': employees[employee_id]}
      else:
          return {'message': 'Employee not found'}