Story points should be independent of the particular individual assigned to that story. Story points are not meant to be equated directly to implementation time. Therefore, the points should not change depending on who is assigned. Rather, you might say something like this:

Senior Engineer is capable of undertaking 30 story points during a sprint; Junior Engineer is capable of undertaking 10 story points in the same sprint.

Although one concept of Scrum might be that all team members are capable of doing the work on the backlog, it doesn't mean that they are all "equally" capable. By extension, it doesn't mean that all team members should undertake the same number of story points during a sprint.

Story points should be independent of the particular individual assigned to that story. Story points are not meant to be equated directly to implementation time. (In fact, some Scrum teams use non-numerical story size for this reason.) Therefore, the points should not change depending on who is assigned. Rather, you might say something like this:

Senior Engineer is capable of undertaking 30 story points during a sprint; Junior Engineer is capable of undertaking 10 story points in the same sprint.

Although one concept of Scrum might be that all team members are capable of doing the work on the backlog, it doesn't mean that they are all "equally" capable. By extension, it doesn't mean that all team members should undertake the same number of story points during a sprint.