There are a lot of handy options for doorbells these days. Many of them are ideal for seniors. The doorbell should have more than a couple volume settings. It's best if the bell or chime can sound in various rooms of the home. Doorbells that come with a variety of tones, such as high and low allows the bell to be adjusted according to the hearing needs. Some have extended sounding times to give seniors a chance to really hear the bell. If the senior is extremely hearing impaired, then strobe light doorbells are a good solution, provided they are installed in a room where the senior spends most of his/her time. Another option is to wire some or all of the house lights to the doorbell so they can flash when the doorbell is activated.
Including a doorbell with a video option is probably the safest option for seniors. You can view who's at the front door from any location globally and some have motion detectors to send you an alert. Many offer night time easy viewing with infrared technology. The images are captured in a virtual file for future viewing.