As we've seen, high miles isn't the kiss of death--not that it was 20 years ago, not with decent maintance. Age though, at least in the rust belt. The more rust, the more I don't want it. Paint is nice, but since I spend my time inside the car I and we have lots of rock chips up here I tend to not care about exterior looks. Interior I care more about, but as long as most of the lights work, eh, point a to point b. Get rid of when I no longer trust the vehicle to spur of the moment cross-country trips, or feel like I can no longer hand off to friends and family if they need to borrow. [Well, the wife's car anyhow.]
For some reason I absolutely do not want to deal with brake lines. If I were vehicle shopping and saw rusty lines I'd keep walking. It probably was foolish to get rid of our Civic but I thought the rust under the hood was starting to pile up and was too lazy to crawl under to see what it looked like underneath (plus I wanted side airbags and ABS anyhow for the wife&kids).