Well, it depends on where you draw the "boundaries" of your Domain and how flexible you want to be with respect to serving various use cases. In any case, the fact that the objects share their lifetimes does not necessarily mean they are better aggregated together.
It looks like your
CustomerSettings object is not really a Domain-related entity, in the sense that it does not represent an actual entity, nor is it related to the Customer outside the specific use case. It appears to exist only because the Customer is the subject of a specific service, which appears to relate to preferences with respect to a notification system, the appearance of a "front end"-like interface etc. and you need a place to persist service-specific preferences.
Consider the concept of a Customer, instead. If, along the course of the evolution of your design, another service comes up, which requires different types of settings, for example a crediting system, where customers choose how to manage their credit. Do you create a new
CustomerCreditSettings object inside the aggregate root?
What I am trying to say is that if you want a Customer object to serve different purposes, and potentially persist Customer objects to a database, it makes sense to only persist what is congruent to the Domain, i.e. your Customers. Whether the database is used by one service or another, which have different needs, expose different capabilities/features and, as a result, require the additional persistence of different "settings", should not matter at all to the Customer object, i.e. you do not make special provisions in your Domain representation just to serve a specific use case. All that because if you incorporate your "Settings" inside the Customer for services that depend on Customers, whenever you are asked to serve an additional use case, you will have to change your basic underlying implementation and this is not a very maintainable solution.
In short, the settings you are referring to (and any related objects not directly contained within the Domain) are probably best used decoupled from the Customer object (in fact, the Customer object can be completely ignorant with respect to them). These can probably be better represented by a map between Customer and CustomerSettings objects, and an observation of corresponding domain events of Customer insertion/removal, which is to be followed by, among other things, a creation/deletion of the corresponding CustomerSettings object, somewhere close to the service. You could take care of CustomerSettings persistence by, potentially, linking it to some unique Customer ID when outside the boundaries of your Object-Oriented Model (i.e. somewhere in "permanent storage world").