Click the Variable View tab at the bottom of the Data Editor window.

Select the Missing cell in the Age row and click the button to open the Missing Values dialog box.

In this dialog, you can specify up to three distinct missing values, or a range of values plus one additional discrete value.

Select Discrete Missing Values.

Type 999 as the missing value and leave the other two boxes blank.

Click OK to implement your changes and return to the Data Editor.

Now that the missing data value has been added, a label can be applied to that value.

Select the Values cell in the age row and click the button to open the Value Labels dialog box.

Type 999 for the Value.

Type No Response for the Value Label.

Click Add to have this label added to your data file.

Click OK to implement your changes and return to the Data Editor.