Validate date of birth in Flutter

A naive approach would be to just construct a DateTime object from the selected birth date and to then compute DateTime.now().difference(birthDate).inDays / 365. That doesn't account for leap days, and maybe it's close enough, but it's not how a human would compute age.

When attempting to solve a programming problem, one of the first things you usually should ask yourself is: How would you solve this without a computer?

To determine if someone is at least 16 years old, you would take the current date, subtract 16 from the year, use the same month and day¹, and see if their birthday is on or before that date, ignoring the time. So just do that:

extension IsAtLeastYearsOld on DateTime {
  bool isAtLeastYearsOld(int years) {
    var now = DateTime.now();
    var boundaryDate = DateTime(now.year - years, now.month, now.day);

    // Discard the time from [this].
    var thisDate = DateTime(year, month, day);

    // Did [thisDate] occur on or before [boundaryDate]?
    return thisDate.compareTo(boundaryDate) <= 0;
  }
}

void main() {
  // The results below were obtained with 2022-06-11 as the current date.
  print(DateTime(2006, 6, 10).isAtLeastYearsOld(16)); // Prints: true
  print(DateTime(2006, 6, 11).isAtLeastYearsOld(16)); // Prints: true
  print(DateTime(2006, 6, 12).isAtLeastYearsOld(16)); // Prints: false
}

¹ This should be fine even if the current date is a leap day since DateTime will convert February 29 into March 1 for non-leap years.

With a function for calculate :

class _WelcomeScreenState extends State<WelcomeScreen> {
  TextEditingController dateinput = TextEditingController();
  final formKey = GlobalKey<FormState>();
  String name = "";

  @override
  void initState() {
    dateinput.text = ""; //set the initial value of text field
    super.initState();
  }

  int calculateAge(DateTime birthDate) {
    DateTime currentDate = DateTime.now();
    int age = currentDate.year - birthDate.year;
    if (birthDate.month > currentDate.month) {
      age--;
    } else if (currentDate.month == birthDate.month) {
      if (birthDate.day > currentDate.day) {
        age--;
      }
    }
    return age;
  }
}

And in your TextField :

GestureDetector(
    child: TextFormField(
        style: TextStyle(color: Colors.white),
        controller: dateinput, //editing controller of this TextField
        decoration: InputDecoration(
            labelStyle: TextStyle(color: Colors.white),
            icon: Icon(Icons.calendar_today),
            iconColor: Colors.white, //icon of text field
            labelText: "Enter Date Of Birth" //label text of field
        ),
        readOnly: true, //set it true, so that user will not able to edit text
        validator: (value) {
            if (calculateAge(DateTime.parse(value)) < 16 || value.isEmpty) {
                return 'Please enter date.';
            }
            return null;
        },
        onTap: () async {
            DateTime? pickedDate = await showDatePicker(
                context: context,
                initialDate: DateTime.now(),
                firstDate: DateTime(1900), //DateTime.now() - not to allow to choose before today.
                lastDate: DateTime(2040));
            if (pickedDate != null) {
                print(pickedDate); //pickedDate output format => 2021-03-10 00:00:00.000
                String formattedDate = DateFormat('dd-MM-yyyy').format(pickedDate);
                print(formattedDate); //formatted date output using intl package =>  2021-03-16
                //you can implement different kind of Date Format here according to your requirement
                setState(() => dateinput.text = formattedDate);
            } else {
                print("Date is not selected");
            }
        },
    ),
),