This is who I use as a measuring stick of good UI design. Until Ive took over, you could give your grandmother or a 5 year old an iPhone and iTunes, and they could both take one look at it and instantly understand it (even if the back end file mgt was a bit of a mess). Functions were on a single screen, logically organized with clear iconography, advanced options were in one place and comprehendible. This was always a big advantage Apple had over windows.
Now try it. They get confused, lost, frustrated, give up and never use their birthday present. No point in buying them expensive fancy paperweights.
I have never found this to be true with any system. The learning curves for some are easier than others, but to someone who has basically NEVER used a computer or smart device, there is some adjusting. On top of that there are some people who want a walk through and people who will spend all night messing with things. (Little kids are almost always an order of magnitude quicker and figure out "intuitive" things that much older people just miss. This is to be expected given the way each groups' brains work.)
I finally got my mom to start using a computer a couple years ago and she had some basic familiarity with them—typed a few emails on my dad's iMac and, oddly enough, used a Linux laptop like 10 years ago to work on a project over a summer. It was mostly just emails and typing notes. So, she had a few things down: keyboard, mouse, and basic clicking. However...
Steve Jobs got a lot of flack for the one button mouse, but you know how many times I had to deal with this: "The right or the left button?" I was asked this constantly. It's such a natural thing to me but there's nothing actually inherently intuitive about mouse buttons for context. Force touch, for instance, isn't inherently intuitive and I actually thought it was sort of gimmicky when it was released. Now I get really annoyed using phones without it. Should it not exist because it's not "intuitive."
Still, Yosemite was her first full time OS on my old 13" MacBook Pro (post Ive) and she did just fine with a few hiccups. The machine is a paperweight now, but because it died. Her first iPhone is my old 5S running iOS 10. Again, I had to spend a couple 15 - 20 minute sessions with her explaining things but she can get around using a smart phone now. Again, post Ive. She got it.
I've had no real problem plugging people in pre or post Ive design. I refused to even use iOS for much of anything before iOS 7.
My dad's first Mac ran Snow Leopard. He did fine too. I really don't find the two interfaces strikingly different aside from the aesthetic. He didn't have issues transitioning. Come to think of it, I've never really had anyone struggle on a Mac, which is why I continue to recommend them.
Oh, and as for 5-year-olds... my two-year-old gets around my phone and iPad just fine. Skeuomorphism—a la the pre Ive layouts—is usually pretty useless for little kids and it's going to be useless going forward because where people used to use a date book, people just use their smartphone now. "clear iconography" to a lot of people still means a floppy disk on the save button. That's pretty clear to me, how about to someone half my age?