I would argue it's not even old people. Most people do not have any understanding of what's going on when you click a button. Website either acts as expected, or it doesn't "work"

If the button doesn't work, the average user is going to say "this most be broken" and then use a competitor (or contact your support). That's why it's really important to error-proof one's design (eg https://en.wikipedia.org/wiki/Poka-yoke).

So instead of the button failing because you didn't check a box, pop up with a message telling them "Please click $box before continuing". Or if you want to be fancy, feed them whatever form you're giving them piecemeal, so that they can't continue until they finish this small part (e.g., have them input a name, then the next page only has a spot for an address, then the next page only has a spot for card information, then the next only has a spot to select shipping). Simple bite sized chunks anyone (well, anyone you would ethically want to sell to) can understand.

Yep, I changed town a few years ago and my new social circles are mostly like that.

Either it works right away without any further questions, or they'll not do it.

Sadly, also if they can't do it on their phone, they will not do it. It's actually very hard to get people motivated to do anything that has to do with sitting down at an actual computer anymore. Which is a bit hard if you're in a very technical political advocacy group, kinda makes me the guy to do everything remotely complex... XD

Yeah, it's really, really not an age issue. If there's an age distinction, it's the range of people who were brought up on computers before the UI was really polished, but even then it's not consistent, and they may not have deep understanding. Kids brought up on iPads, and who aren't forced to learn by their interests or educators, have no clue.