There is indeed an ambiguity whether the DFD level count start as 0 with the context diagram (example here), or the overview diagram below the context diagram (example in this article).
The context diagram shows the process/system and its exchanges with the outside world. At this stage you can’t have a store.
The overview diagram shows the main sub-processes and how the data flows between them. And this can already include data stores (example in this other question or in figure 3 of the academic article above).
So in level 1 (so level 1 or 2 depending on where you start) you can without any doubt have data stores. If you didn’t see data stores at lower levels, it can be because the examples are too simple, or it can be because the decomposition chosen by the authors.
Additional information: The ambiguity in the level numbering is because:
- on one side, it is a popular opinion that it’s about the level of the diagram, so 0 is the very first diagram in the decomposition hierarchy.
- on the other side, it can be about the level of decomposition of the processes in the system. So the main processes are at the top of the decomposition level.