I'm going to tentatively suggest that there is no single word in English to describe "people's assumption that their bodies/minds are absolutely going to get worse as they age." I think the minimal expression for this is going to be a two-word phrase. With that in mind, you could pair up one of each of these adjectives and nouns as you see fit:
anticipated/expected | senescence/decline
Personally, I prefer "anticipated senescence."
senescence1: the state or process of ageing, especially in humans; old age
I drew on this source for my answer, particularly the following passage (emphasis mine):
Since social and individual expectations anticipated decline in old age, and since one implicit way of thinking about a disease was that it limited what was considered normal functioning, then [senile dementia] would not be considered a disease.
[Source: Health, Disease, and Medicine: Concepts in Medicine by Arthur L. Caplan, James J. McCartney]