Of course it is. Every fiber of her body is telling her that the 91 year old is non-viable and repugnant.
Every fiber of her body is telling her that the 19 year old is not only viable but yummy.
The flat fee is basically the 19 year old subsidizing the 91 year old for the immense effort that the escort has to make to override all the red alarms going off in her brain, heart and gut about having sex with a 91 year old.
On the contrary all the indicators are green when she's with the 19 year old but he has to pay the same rate of the 91 year old.