Seniors can't or won't walk very far. While buses are more flexible than rail in this regard, neither work for seniors in suburbia. Suburbia is incompatible with mass transit. Seniors want to be able to board and alight within 2 or 3 blocks of their terminus, maximum. Until we get automated cars, the only way to achieve that economically is with density.
I often tell people that San Francisco is the perfect place for seniors.[1] Bus stops are usually a maximum of 3 blocks apart, sometimes only 2 or 1. And there are enough bus lines that there's at least one bus stop within a 2 block radius for the vast majority of residences, and often multiple stops for multiple lines within 2 blocks. In other words, you can pretty much get from any place to another other place without walking more than a few smallish blocks. Whenever MUNI tries to reduce the number of stops, irate and politically powerful senior citizens flood the hearings.
I used to live on a particularly maddening segment of the 3-Jackson line that literally had 4 regular stops on 2 blocks (5 stops in 3 blocks): https://www.sfmta.com/routes/3-jackson Not coincidentally that stretch had two large housing projects--one exclusively for seniors and another predominantly for seniors--directly on the bus line, plus a population that skewed older more generally. The 3-Jackson line has at least 1 stop for most every block; the line averages about 1 stop per block! (I counted 29 stops in 31 blocks inbound.)
For this and many other reasons, San Francisco is an amazing place for senior citizens, rich or poor. (There's a ridiculous number of senior housing and community centers, mostly invisible to people not in the know.[2]) But the mass transit system sucks for working commuters because the system is biased towards the needs of the elderly.
[1] And I mean it. I cajoled my mother to move here in 2011. She finally sold her car last year. For awhile she worked at a community center that provided free lunches to seniors, where several very wealthy seniors from Marin commuted into the city to eat alongside and converse with very poor seniors, simply because they wanted and needed the company.
[2] Interestingly, San Francisco is (or was) being sued by HUD for racial discrimination. The supposedly most liberal large city in the country had a housing system that segregated people by ethnicity--senior housing projects tended to be Russian, Chinese, Latino, black, white, etc. Except for black areas, this was pretty much self-segregation. I don't know the extent to which the city perpetuated it, but they didn't seem to try to do anything to change it. (There's a waitlist system for senior housing and the seniors, at least, seemed to assume that people of the appropriate ethnicity were bumped, depending on the neighborhood. But I think mostly it simply had to do that seniors--and people generally--find their way to public assistance through their social networks, and different social networks will tend to channel people to particular programs and outlets.)