1.Icons are only useful for users that already know what they do. Zero value UX wise for new users. Labels matter, especially on mobile where hover is not an option. Having a "walkthrough" on first load is not a replacement. Also, non-native users do not know about hover tooltips, and they will not learn that behavior, they find them almost exclusively by accident.
2.Gradients on mobile, especially with power save, make apps and pages unreadable for everyone, but especially seniors.
3.Hamburgers and triple dots are not visual indicators for non-native mobile users.
4.Swipe is not an replacement for any UX process for non-native mobile users.
5.Canceling/backing out of any workflow is rarely obvious on flat/minimal designs. Seniors click/tap the wrong thing ALOT. Some giant percentage of their time is spent trying to reverse an action.
6.Zoom is king. Non-button looking buttons get even worse if you remove their context. Mouse over only context buttons might as well be invisible.
7.Seniors do not know what blue/purple/green text means. The underline is the only thing my seniors understand. Also, they click pictures, all pictures, because they are the only thing on these apps/pages that actually look like they are clickable.(see number 6 buttons)
If the native UI toolkit is a language the my phone is the tower of babel. And I hate that with passion and fury.
"Click the hamburger icon to open the menu."
"The three parallel lines; I agree, it makes zero sense, but now you know."
"People are calling those a hamburger"
"Cool. I see it, buns and a patty. What next?"
"It's signage for the thing you do to get a menu of options and settings. Now click the hamburger...."
That's how I have always done it. Bring things up with context so they are always getting a little bit of UX culture along the way.
Frankly, I hate flat design. The visual hints are a very good thing, and I have found explaining them once multiplies through all future interactions.
This crap requires a lot more fundamental explanation a lot more of the time.
As far as I'm concerned, flat is a regression. Maybe it's more efficient, but just in machine terms. It's not in human terms.
I've tried; three parallel lines in a flat UI do not indicate "menu" to a great many users; including myself, at one point in time.
Also, I'm a programmer in my 20s and I get confused by mobile apps all the time.
Material design is flat I think and is very good. I'm sure if we stick with it, it will be very obvious to those who will be old with me. What can we do to make it useful for old folks today?
I think this simply boils down to people having to learn how things work, and being able to adapt to the form factor being used. Be it mobile, tablet or desktop. There are UI conventions across platforms although that gap is being bridged somewhat (many desktop websites have hamburger menus now, for example).
Software changes and so does UI, not much that can be done except better education, otherwise we'd still be looking at the ancient windows desktop interface...