Use [CODE][/CODE] tags around your code to make it more legible.
The problem is here:
yrd = ( 2012 ) - yr ; //'yr' has no value!
cout << "Enter your year of birth:" << endl;
cin >> yr ;
You're running the line to set yrd before you've assigned a value to yr, so you're going to get unpredictable behaviour! A good habit to get into is always assigning a value to variables before using them, and that way you will at least have a predictable (read: more easily found) bug.
EDIT: oops, beaten by crabboy... Sorry