Switch: Multiple values in one case?

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.

You have to do something like:

case 1:
case 2:
case 3:
//do stuff
break;

In C# 7 it's possible to use a when clause in a case statement.

int age = 12;
switch (age) 
{
  case int i when i >=1 && i <= 8:
    System.Console.WriteLine("You are only " + age + " years old. You must be kidding right. Please fill in your *real* age.");
    break;
  case int i when i >=9 && i <= 15:
    System.Console.WriteLine("You are only " + age + " years old. That's too young!");
    break;
  case int i when i >=16 && i <= 100:
    System.Console.WriteLine("You are " + age + " years old. Perfect.");
    break;
  default:
    System.Console.WriteLine("You an old person.");
    break;
}

1 - 8 = -7

9 - 15 = -6

16 - 100 = -84

You have:

case -7:
    ...
    break;
case -6:
    ...
    break;
case -84:
    ...
    break;

Either use:

case 1:
case 2: 
case 3:

etc, or (perhaps more readable) use:

if(age >= 1 && age <= 8) {
     ...
} else if (age >= 9 && age <= 15) {
     ...
} else if (age >= 16 && age <= 100) {
     ...
} else {
     ...
}

etc