I’ve been doing this by stacking three question elements on the same page, for example:

1. iQ_Text] -> a  ‘text/graphic’ question that says “1. How old are you?”
2. �Q_Num] → A ‘text entry’ question where respondents can type in a number (You could use CSS to hide the ‘question text’ for this, or put in an instruction like “Enter a whole number”)
3. hQ_PNTS] → A ‘multiple choice’ question with
   * “Prefer not to say” as the only answer option, and
   * ‘Answer type’ set to ‘Allow multiple answers’ (so that respondents get a checkbox instead of a radio button that can’t be de-selected)

To make sure that participants answer either TQ_Num] or eQ_PNTS] but not both, I usually use in-page display logic, but you could probably enforce something like that with custom validation too. My display logic works like:

• tQ_Num] only displays when “Prefer not to say” is not checked-off, and 
• Q_PNTS} only displays when Q_Num is not empty. 

Hello ​@TerryFoster 

You can try Using Form Field with Two Choices.

1. Add a Form Field question.
2. Create one field for Age (Text Entry) and another for Prefer not to answer (Single-Choice Checkbox).
3. Use Display logic or Custom Validation to ensure only one is filled at a time.

​@TerryFoster,

I would use a multiple choice with allow text on the first choice and prefer not to say as the second choice.