input: Write a python function to calculate age given birthdate and current date.
  assert calculate_age(datetime.date(1996, 12, 6), datetime.date(2021, 3, 18)) == 24
  assert calculate_age(datetime.date(2000, 1, 1), datetime.date(2021, 1, 1)) == 21
  assert calculate_age(datetime.date(1980, 6, 15), datetime.date(2020, 6, 14)) == 39
  assert calculate_age(datetime.date(2005, 5, 25), datetime.date(2023, 5, 26)) == 18
output: def calculate_age(birthdate, current_date):
      years = current_date.year - birthdate.year
      if (current_date.month, current_date.day) < (birthdate.month, birthdate.day):
          years -= 1
      return years