Well, yes it does make assumptions based on age
That is the problem. It confounds "age" with "resistance to change" or "stupidity" or "low technical ability" or some such characteristic for which the explanation project might be appropriate.
Presumably the actual goal is "to explain
BTC to people who don't understand it" (of any age).
Using "grandparents" is a distraction from the actual task, and is a gratuitous insult to many people - for absolutely no gain to the project.
Substitute any other generic group descriptor for "grandparents", preferably one that fits your own demographic, whatever it may be, and feel the sting for yourself. If that's not strong enough, imagine stereotypical corny drawings added for demeaning emphasis, along with stereotypical corny dialog attributed to the target group.
Those things hurt, people, and you gain nothing from it in terms of advancing your stated goal of explaining
BTC.