Your first assertion

assert statements invalidate any data where the expression evaluates to false.

For the following data it looks like Jill's age has been accidentally set to the year she was born, not her age. Let's flag this by making sure the age value is less than 100

Add this assertion to the Model:

assert age < 100

Remember to click the "Run" button after you have made changes

Next: Constants

User(id) {
  id: Number
  name: String
  age: Number
}