After further review, it seems that even though Jennifer Lawrence might believe she had an imposter butt in a photo of her in a bikini, the “90-year-old butt” is indeed her very own toosh.
A close up of the photo in question:
This is the original, undoctored, straight-from-the-photo-agency version:
Here is the “90 year old butt” (left) with the real butt (right):
Here is another photo of her butt taken about one second after the first photo, where it looks basically perfect, leading us to believe the first butt was just an unflattering moment that got captured.
Conclusion: The butt that Jennifer does not want to take credit for is indeed her butt.
Conclusion #2: It doesn’t really matter because Jennifer Lawrence is still perfect in every way and LOOK AT THAT BOD!