You can't specify a range in the case statement, can do as follows.
case 1:
case 2:
case 3:
case 4:
case 5:
case 6:
case 7:
case 8:
MessageBox.Show("You are only " + age + " years old\n You must be kidding right.\nPlease fill in your *real* age.");
break;
case 9:
case 10:
case 11:
case 12:
case 13:
case 14:
case 15:
MessageBox.Show("You are only " + age + " years old\n That's too young!");
break;
...........etc.