* Nature can’t just choose a cutoff time for death – Menopause would be one of several biological factors to influence a guppy’s lifespan, and might be offset due to the reduced demands of producing fry.
* Perhaps in the wild, they DO die off more quickly, due to their increased size and visibility. In an aquarium setting where they’re comfortable, well cared for and pampered, they just hang out and look pretty.
* I’m guessing that the evolutionary factors actually slightly favor a female that lives in the wild until Menopause. After all, she’s smart enough/quick enough/patterned well enough that she managed to have lots of rounds of fry, therefore more of her genes went into the future breeding pool. The longer they remain fertile, the larger their batches of fry get, as well, so if they lived long enough to reach menopause, they had LOTS of batches, probably a few really big ones. So, even though the menopausal female doesn’t actually play Grandma and watch the babies for Ma, she does have lots of offspring hanging around, reproducing with 1/2 her genetics, so it makes sense those fry also have her survival characteristics.

That’s all I can come up with for now, I’ll add more if I have any other smart sounding thoughts!