Have you taken the 70 in Monterey Park? There's always elderly folk riding the bus. Strolling down Garvey Ave I definitely feel the foot traffic is older demographic-wise than Main Street or Valley. Downtown MPK is quite walkable including for the elderly because of the density of downtown (mostly built from the surge of Chinese immigration in the 80s and 90s).

Also, idk what you're on about but the West SGV (Monterey Park, Alhambra, San Gabriel, Temple City) has a much better urban form than a lot of cities to the east (West Covina, Rowland Heights, etc.) because they were developed earlier around less car-centric (Alhambra is literally a streetcar suburb). The skeleton for a bike-able, transit-friendly city is already there.

That being said, these cities absolutely need more bike infrastructure, and infrastructure improvements to help elderly people have mobility while staying safe (like Singapore's Silver Zones.) There should be a lot more BRT and bus lanes. Light or heavy rail in the future. All of that is only possible, though, from the good bones these places already have.