Function to use substring & calculate ageRange

calculateAge.dart

calculateAgeRange() async {
    FirebaseAuth.instance.currentUser().then((user) async {
      final snapShot =
          await Firestore.instance.collection('User').document(user.uid).get();
      User userSnapshot = User.fromSnapshot(snapShot);

      if (userSnapshot.AgeRange == '') {
        if (userSnapshot.DateOfBirth == '') {
          print('DATE OF BIRTH IS NULL');
        } else {
          String ageRange;
          int picked = int.parse(userSnapshot.DateOfBirth.substring(6));

          DateTime currentDate = DateTime.now();
          var age = currentDate.year - picked;
          if (age > 18 && age <= 25) {
            print('age range is 18 - 25');
            ageRange = '18 - 25 years';
          } else if (age > 25 && age <= 29) {
            print('age range is 20 - 29');
            ageRange = '20 - 29 years';
          } else if (age > 29 && age <= 35) {
            print('age range is 30 - 35');
            ageRange = '30 - 35 years';
          } else if (age > 35) {
            print('age range is 35 and above');
            ageRange = '35 & above';
          }
          print('AGERANGE: ' + ageRange);
          profileBloc.ageRangeSink(ageRange);
        } 
      } else if (snapShot.data.containsKey('AgeRange') == false) {
        print('AGE NOT FOUND');
        return null;
      } else {
        print('AGE RANGE: ' + userSnapshot.AgeRange);
      }
    });
  }

Works !