I think the relationship people are seeing is "80 year old can't fill out my form, must be because they can't use a computer." But international surveys like PIAAC [0] indicate that adaptive problem solving is the real problem. 30 percent of adults are at or below Level 1; I've copied and pasted what that level represents below:

> Adults at this level are able to understand simple problems and develop and implement solutions to solve them. Problems contain a limited number of elements and little to no irrelevant information. Solutions at this level are simple and consist of a limited number of steps. Problems are embedded in a context that includes one or two sources of information and presents a single, explicitly defined goal.

This test is administered on a tablet, so I think scores can be interpreted as a sort of combo of computer use and problem solving.

A full 40% of adults from 55-64 are at that level or lower. Wikipedia thinks that a novel online form would be a Level 2 task, provided it involves navigating across more than 1 page. Based on that framework and assuming your dad can use the sort function in his email, he represents a top 20th percentile adult for the 55-64 age group. It's probably even higher considering his age as the trend is towards older groups having a rougher time with the survey.

I also think it's ageist to assume that older people can't use a computer. But assessments like this indicate that a full quarter of the US regardless of age would have trouble with some of the basic tasks we associate with computer use. So designers should consider their intended audience when deciding what's simple or not simple enough.

[0]: https://nces.ed.gov/surveys/piaac/2023/national_results.asp