On a language teachers’ list, the comment was recently made:

[for] mature adults … the transition from TPRS to TCI … may occur much earlier than in most high school students.

I have not found that the transition from TPRS being the tool of choice to general TCI being the best choice occurs earlier with adults — in fact, in my experience it may occur later because adults just have more stuff going on and because of external factors don’t “get it” as easily sometimes.

I think it is very important to draw a distinction about what is meant by TPRS. It doesn’t mean stories about green cats — necessarily. That could be one way to go. But the bones of TPRS is repetitive input that is kept at 100% comprehensibility, and students that are held accountable for comprehending 100%. It doesn’t matter if it’s conversation or a story about the person’s dad or a wild tale about a hedgehog. The content doesn’t matter. The language — and particularly how the language is presented to the student — does. But everything centers around establishing meaning through a robust means — the shared native language — and providing very high levels of repetition.
TCI simply means teaching using comprehensible input — close to or at 100%, not the sort of kinda-sorta comprehensible input that is so fashionable these days among task-based or communicative language teaching circles. TCI still means establishing meaning through a robust means such as the shared native language, and giving high levels of repetition.
After awhile, at the higher levels, there is enough language in general that things shift from a need for TPRS to TCI. I don’t think that generally the number of reps needed to acquire a structure necessarily drops, but the number of reps needed to solidify an item (an example of that structure, dressed in certain vocabulary words) seems to. That makes sense, because students are able to leverage the language that has already been acquired to comprehend. We are not usually measuring acquisition during class, in formative assessments like comprehension checks and quizzes — we are measuring comprehension. They’re not the same. Comprehension can be built more quickly when someone has language to use.
Adults are sort of like false beginners. They feel OK with having language rattling around in their heads. But that’s not acquisition. That’s as far as they’ve ever gotten before using traditional methods, so they feel like they ought to be using the same amount of time and things will be fine. That might be OK for vocabulary, but not for structure. And the vocabulary gains will not be long-term unless they are eventually underpinned with “enough” repetition (whatever that number is) to cement them as acquisition.
It could be partly a need on the part of adult students to feel they’re doing something “serious” with their language. I doubt their need is to get away from TPRS techniques themselves. If the engagement is not an issue (i.e., they would be willing to listen to you read the phone book if they thought it would improve their language) then you don’t need to worry about using the elements of TPRS that are designed specifically to make wiggly adolescents sit still long enough to listen to enough CI to acquire (personalization, story-asking) and can replace those with factual or serious content at the right level.