When I first discovered Walkscore, I "fell in love" - until I realized that topography was (essentially) ignored (except from bicycle perspective). Nonetheless, I rely upon Walkscore as first measure of likelihood of zero-to-minimal need for a vehicle. However, a 95+ Walkscore can still leave some individuals nearly as stranded as if in a low walkscore area (one identified as requiring a vehicle).
There are *multiple* "categories" of people for whom hills (even quite mild hills, whether up or down) make a place NOT walkable. Many of these categories are ones which "can happen to anyone" (as through an unexpected injury).
Walkable (by popular definitions) is a start, but is a grossly incomplete solution, unless the goal is merely to make a place more interestingly-accessible to people who are already "athletic" and /or "outdoorsy" - a large percentage of whom are already enamored with walkable cities, buses, bicycles, etc. (some, for ethical reasons) and/or people whose only interest is locating a place to live that may allow them to walk to where they work (as if workplace is both long-term predictable and, also, the only important criteria). See .... (if I can locate the source I seek, I'll edit or add further comment)
Most people don't have the freedom to relocate to a more suitable place. The places need to become more suitable to the residents. And the transit needs to be inviting to tourists, as well. "Renting a car" should not need to be an automatic next-step after arranging airplane travel, even (especially?) in emergencies.
Also, not all people thrive (or can learn to tolerate or function) with the noise-levels, over-stimulation, and other aspects of (contemporary) urban living. Then, there's the air-quality.
Please explain why/how someone who is interested in "Aging in Place" would be best in an urban environment (unless they want extraordinary access to "Arts and elaborate restaurants" - beyond what can be found in many "small" communities.