This and other similar questions keep coming up so to flesh the response out a bit ...
Set the Start conversations permission for the Registered user group to No
Create a new user group - let's call it Established members
Set the Start conversations permission for the Established members user group to Yes
Create a User group promotion to promote members to the Established members user group using the User has been registered for at least X days criterion and set it to 7
The same approach can be used for other permissions and criteria.
Always remove the permission from the Registered user group and add it to a new user group, and combine it with a promotion.