using System;

​

public static class Program

{

    public static void Main()

    {

        var birthday = "";

        var _birthday = DateTime.ParseExact(birthday, "yyyyMMdd", null);

        Console.WriteLine(_birthday.HowOld());

    }

​

    public static int HowOld(this DateTime birthday)

    {

        var nowAt = DateTime.Now;

        var nowYear = nowAt.Year;

        var nowMonth = nowAt.Month;

        var nowDay = nowAt.Day;

        var birthdayYear = birthday.Year;

        var birthdayMonth = birthday.Month;

        var birthdayDay = birthday.Day;

​

        return birthdayMonth < nowMonth

            ? nowYear - birthdayYear

            : birthdayMonth == nowMonth

                ? birthdayDay <= nowDay

                ? nowYear - birthdayYear

                : nowYear - birthdayYear - 1

                    : nowYear - birthdayYear - 1;

    }

}