I've interpreted it as meaning: If you create a new app version and want to submit it for review: Before January 31st - the "Add for Review" button will be enabled - even if age ratings is incomplete. After January 31st, if age ratings is incomplete - the "Add for Review" button will be disabled - ie "...you won’t be able to submit app updates in App Store Connect." Complete the age ratings, and then you'll be good to go.

What I wrote was my own interpretation. Apple has not clarified anything for me. The email wording is confusing - as we see from so many people asking for clarification. It's the reason I came to the forum - looking for clarification.

I can confirm this has resolved this issue. I manage 4 developer accounts across 4 brands and can confirm issue is resolved in all accounts. Thank you.