Impossible question to answer intelligently. Some are fit, others not so much. Some live in Florida, others in Colorado. Some have health compromises, others are not there yet with all their bits and pieces still functioning.
What's a senior citizen in your poll? I didn't start riding until I was 64, and now I'm 70 and approaching 16,000 miles on eBikes. I don't care to speculate what others need. What do you plan to do with the data?
For me Electra, Trek Pure, KHS Smoothie, and other Flat Foot frames are the most comfortable for city and multiuser improved paths. Not good if you want to ride MTB trails. Great upright positions for traffic awareness. I personally wouldn’t buy any of them as eBike but prefer to kit and get a better eBike. More info on where you’re riding can get better advice. It’s difficult to recommend a motor type without that understanding.
My wife's sister is in her 70's and small in stature and I purchased a Rad Mini ST as a thanks for helping us on some electrical issues (she's a retired electrician) because she wouldn't take anything for helping. It's small, low and comfortable. She rides it when she can and really enjoys it.
So I would say whatever fits you and your terrain needs.
I personally favor Class 3 Mids. Average people in the US are out of shape. People in their 30's today weigh on average 20-pounds more the people in their 30's in the 1980's. I don't like throttle bikes but for someone out of shape and physically elderly, which isn't necessarily chronological, a low or mid-step light weight bike with a mid-drive and a throttle would likely be their best investment. I had someone this week who didn't have the hand strength to shift and cannot lift a bike onto a car rack. It is sad, she was not chronologically old. Two people both in their mid-forty's can have differences in their telomeres representing a 12 year difference in biological age. This is largely due to early trauma and chronic stress. We live in a traumatic world and people are stressed. Riding bikes is a great way to recreate while getting healthy and it is better for the planet than driving cars or yelling at a TV.
Thanks everyone. I learned a lot reading this thread so now I will take that as an input and feed it into a bike. I picked up the ideal donner bike today. It is a low-step with strong wheels, and internal gears for low maintenance. The riding position is upright and its overall weight will be under 40 pounds. I like small displays and do not like throttles but this bike is not for me. Older people want to be able to see a display and tend to want throttles. I replaced a small display with a large one for a rider today who is in her late 80's. Here is what the donner bike looks like before the transformation. Yes, that is a built in lock and dynamo light system. Thanks again.
I am almost done with the 'adult' bike based on this thread. I purchased it on Thursday and it found a home on Sunday. It will be ready today. The grips are thickly padded and @AHicks advised using a light touch MicroShift shifter for the Nexus inter 8 internally geared hub instead of a twist shifter that is harder to work with arthritis or gloves. I will post a photo when it is done and look forward to comments and advice regarding what I can do better next time to better get it right.