The cheapest service that I've found and currently use is Tello (a former Sprint MVNO). Unlimited text/calls + 1GB data is $10/mo. the plan can be changed on the fly, and no commitment. Downside is you need to bring your own compatible device or purchase one of Tello's refurbished models. The Moto G series works well enough on the Tello network.
I actually get the 500MB plan and only use about 100MB/mo. Turning on Data Saver in Android, disabling nearly all background networking except for essential system services, and sticking primarily to WiFi avoids using network data.
Seniors are generally on a fixed income and therefore can't afford to spend too much per month. So cheap phone plans and cheap phones are a must. I recommend Tello's lower end plans as a middle ground option between Mint Mobile and Ultra Mobile.