Old Lady Reimu?

I will refrain from making a joke, as I feel that this pool is referencing something. Could someone elaborate?

I think that the artist is calling her "old lady Reimu" to indicate that he/she is referring to human Reimu as opposed to yukkuri Reimu.

She's just genuinely old(er) in this story, that line on her face is a wrinkle, and she has slightly saggy cheeks throughout.