String input 값 : “20030603”

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Solution #1

int age(String input){
  DateTime _dateTime = DateTime(
      int.parse(input.substring(0, 4)),
      int.parse(input.substring(5, 6)),
      int.parse(input.substring(7, 8)),
  );
  return DateTime.fromMillisecondsSinceEpoch(
            DateTime.now().difference(_dateTime).inMilliseconds).year - 1970;
}

void main() {
  var input = "20030603";
  print("대한민국 나이(만) : " + age(input).toString());
}

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Solution #2

int isAdult(String enteredAge) {
    var birthDate = DateTime.parse(enteredAge);
    print("set state: $birthDate");
    var today = DateTime.now();

    final difference = today.difference(birthDate).inDays;
    print(difference);
    final year = difference / 365;
    print(year);
    return year.toInt();
 }