Apple Engineer says you must submit a new version in order to update the Age Rating. See above
https://developer.apple.com/forums/thread/796744?answerId=853777022#853777022
I also called Apple Developer Support and they confirmed. Just creating a new version and updating the age rating without submitting is equivalent to doing nothing. Age Ratings must be reviewed by Apple Review.
Topic:
App Store Distribution & Marketing
SubTopic:
General