When people assume that their bodies, etc. are absolutely going to get worse as they age -- is there a word for this? I personally think that people are altogether too accepting of this idea -- and that we should be fighting with every tooth and nail to stay as young and fit as we can be, without adopting the lies and "misleadings" of the media, doctors, and other "authorities."
But anyway, is there already a word for this?
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]
