"When the system launches, all VRChat account users will receive notifications indicating their status in the Content Gating system." So it will affect current accounts, probably based on your recent history.
Also, private rooms = no restrictions. You can be nude, or host a virtual sex toy part. whatever. It's private room only accessible by invite, so age shouldn't be a problem. But, yeah, if you're in a public room, don't do things that violate TOS, that's common sense. The age to play VRChat is 13 and up, the same as even having a Steam account.
This isn't mean to target underage content, it's made to discriminate based on the opinions of other people. You can get banned from uploading content just because a group decided to block/mute/kick you because they didn't want you around. Not necessarily because you did anything wrong.
They don't really say what the conditions are, and they said that they won't be releasing the details about the system. Personally, I feel that is bullshit. And they don't specify whether it will block stuff you already have uploaded.