If the intent of the second set of answers ( through ) will only be requested if the first question is answered as "I don't remember", then I would create question 1 as a short text question, with the answer being optional:
How old were you when you met your car accident [ ]
Then, you create the second question with a condition based on the first question not having been answered. That second question is a mandatory single choice list question, similar to:
Can you give us an idea of your approximate age at the time of the car accident?
0 Under age 17
0 Between 17 and 24
0 Between 25 and 34
0 Over age 34
(in my example above, the "0" character represents a radio button - where you can only select one answer)
Hope that helps.
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