App rejected for age rating, despite having no adult content whatsoever
Requests for changed metadata happen all the time. If after a day or two of careful review of the iTunes Connect FAQ and guidelines, you still aren't comfortable you can always ask for more help.

In iTunes Connect - select contact us. 


For the first option - App Review.
For the second option - App Store Review
For the third option - Ask/Address Review Questions


From your posted answers, you could also go for the 4+ rating and put a parental gate in rather than ask people to confirm they are 13+ before entering user information. The easiest fix might be to not ask for registration at all. Also, if you have any sort of web browser or user configurable internet search lookup or feeds, you’ll want to disclose you have unfiltered access to the internet. 

My suspicion is you’re hung up on section 5.1 privacy. https://developer.apple.com/app-store/review/guidelines/#privacy

In the end, you are correct that it’s your call on which you feel best serves your app - no sense compromising your answers if you’re certain that’s most important.