def validate_initial_digits(credit_card_number: str) -> bool:
    """
    Function to validate initial digits of a given credit card number.
    >>> valid = "4111111111111111 41111111111111 34 35 37 412345 523456 634567"
    >>> all(validate_initial_digits(cc) for cc in valid.split())
    True
    >>> invalid = "14 25 76 32323 36111111111111"
    >>> all(validate_initial_digits(cc) is False for cc in invalid.split())
    True
    """
    return credit_card_number.startswith(("34", "35", "37", "4", "5", "6"))