Re: "Young students, for example, cannot be expected to remember and enter a password. "
I'm not sure that's accurate. In principle, I agree that the stereotype should die. However, let's analyze some things you claimed:
"And of course the entire computer industry and the Internet was developed by people now nearing their allotted four score and seven years."
That puts the computer industry and internet as developed by people born in 1932. Some of them, sure. Most of them, no. The people who did a lot of the modern-day internet technology were born in the 50s through the 80s. We're not including every computer science professor who wrote a lot of important texts; I'm thinking the engineers at the companies who designed the products we're currently using, from old concepts like HTML and HTTP to newer technologies like JSON. However, I also contend that this doesn't matter; if the point is that older people have had less contact with computers, citing old computer scientists is finding the exception that certainly doesn't disprove the rule.
As for when the majority of people encountered computers, I do not think we can really count the machines of the late 70s and early 80s. I don't think they count for a generational rule because they were not that commonly held by everyone. Remember that a lot of people here had them because we self-select to be more interested in computers. The population at large was not guaranteed to have a home computer in 1985. I would conservatively estimate that, if you were a child in 1990, that you would then be guaranteed to have a lot of contact with computers during your youth. We'll say that this would happen if you were younger than ten years at the time.
This puts our threshold of stereotypical computer familiarity birth year at 1980. In other words, the maximum age for such a person is 38 years. Many teachers are older than this,, as it is a job they typically hold for many years. Since we're talking about primary and secondary schools, I estimate that about a half of my teachers were above the age of 40. Nearly all the teachers were older than 40 in my primary school, though I do not know if that is a pattern.
Of course, this is a stereotype, and will not be generally correct, but I believe I've made clear that there are many people who did not have contact with computers during their youth. There is no guarantee that, even though they have undoubtedly had to use a computer at some point during these past decades, that they are literate in the technology and can successfully manage it. Look at all the people that are, according to this stereotype, supposed to know what they are doing. Many of them are not competent in using it. Unfortunately, while I have found many older people who have no difficulty with technology, I have found many more who reject it entirely or make me wish that they did.