Arizona also allows people under 30 to drive, and every age group under 30 has a much higher accident rate than 80+. People under the age of 18 have quadruple the accident rate of people aged 80. The reason people incorrectly believe that octogenarians are bad drivers is they have a tendency to die in even minor accidents.
I think folks think 80+ drivers are bad, is they drive conservatively and carefully. Its annoying to 20-something drivers who want to race around and take chances.
See, the thing is, Waymo controls the data, and certainly knows how good their driving is. If it was clearly better than the average octogenarian driving, wouldn't they be letting us all know that? Wouldn't they be announcing that and writing up white papers and giving presentations at conferences about this? They need to start building the safety case, and it is entirely to their benefit to release this data, if it truly is more reliable than even just the average octogenarian. The fact that they don't, and so far as I can tell only release the minimum data that they are legally obligated to (and marketing-speak like https://waymo.com/safety/ is not the same thing at all), leaves me with an inference that the data is not nearly that rosy.
There's a difference between something being safe and being capable. I can make a trivially safe self driving car that just sits there and does nothing, completely safe but not capable :) My point being that Waymo could have zero colissions (which I believe is not true) but still not be as capable a driver as someone for which it is unsafe to be driving, but is reasonably capable of getting around.