On July 17th 2018, ACTIVEWorks Endurance has released a new feature to support form questions for each registrant type - Myself, Someone else below 18 and Someone else above 18.
For all events, three separate form question tabs (screenshot as below) will be provided at Create Form Questions:
When creating a custom question, you can add this question to the other two "Someone else" forms by check the box of Also add the question to then select all of the available options.