Here she is explaining to Dave that she is NOT the owner of the butt in a photo taken of her while in Hawaii:
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!