datetime

main.dart

class Person {
  Date birthdate;
  
  int get age => new DateTime.now().difference(birthdate).inYears;
}

void _main() {
  var p = new Person()..birthdate = new DateTime(1978);
  
  print(p.age);
}

main() {
  try {
    _main();
  } catch (e) {
    throw e;
  }
}