WPF DataValidation and SaveCommand

I have a DelegateCommand (SaveCommand) that has datavalidation performed in a CanSave method.

I have an integer Age property on a ViewModel.

DataValidation as follows:

      private void ValidateAge(int ageValue, bool throwException)
        {
           
            if (ageValue <= 20)
            {
                this.AddError("InvalidAge");
                if (throwException)
                {
                    throw new Exception("This is the exception thrown from ValidateAge method");
                }
            }
            else
            {
                this.RemoveError("InvalidAge");
            }
        }

The validation is called every time the Age property is updated, and the Save button is appropriately enabled/disabled ..... except in one situation:

If I delete all text from the TextBox used to input the age value, and then move the focus to another control, a red border surrounds the Age TextBox and the validation method is not called: this leaves the Save button in an enabled state, and when pressed calls the SaveMethod.

What am I doing wrong. This does not happen with string properties (the Save button is appropriately disabled if all text is removed from say the FirstName TextBox)