int year, month, day;
Console.WriteLine("Please enter your birth year : ");
year = int.Parse(Console.ReadLine());
Console.WriteLine("Please enter your birth month : ");
month = int.Parse(Console.ReadLine());
Console.WriteLine("Please enter your birth day : ");
day = int.Parse(Console.ReadLine());
Console.Write("You are ");
Console.Write(CalculateAge(year, month, day));
Console.Write(" years old.");
Альтернативный способ:
Console.WriteLine("Please enter your birthday in the format YY-MM-DD : ");
string line = Console.ReadLine();
string[] parts = line.Split(new char[] { '-' });
int age = CalculateAge(parts[0],parts[1],parts[2]);
Console.WriteLine("You are {0} years old.", age);
И -PLEASE- выполнить некоторую проверку ввода.