(note I believe this is not a duplicate of Would relatively primitive people really confuse technology with magic? - that person's question is if there were precedent for primitive people finding modern technology for their era magical, I am asking what the era would be where someone would start not even being able to understand what modern devices were used for at all).
Looking for the earliest era I could have someone be able to travel from to the present day and still be able to sort-of-function on their own with modern technology.
Assume the person is otherwise normal and basically educated for their era, and the location is relatively similar to their original place of residence (farmer in a rural area, a city dweller on a street, etc). Define function as within the first 12 hours of arriving in a location find shelter, feed themselves, get some rest, tend a minor wound and start trying to figure out where the heck happened without being so mystified and overwhelmed at all of the things and noises and sounds or unable to understand their surroundings.
For example, if someone came to the present from 1965 they would have a lot to learn about modern politics, etc and the capabilities of our devices, but they would already know what the devices' original intent and use cases more or less well enough to operate in a modern residence. They'd know the refrigerator held food, the device on the wall turned the lights on, the screen in the corner was probably some kind of television, etc. They would know the big box with wheels outside was a car was and more or less how to operate it, what the planes going through the sky were and what they were doing, etc.
Someone from 1945 would be able to do the same, though the television and phone might be a mystery to them but if they somehow accidentally activated it they'd be able to map those devices into a functional model - the tv is like a radio with pictures, the phone has numbers it, maybe I can use it to call other phones, etc.
Someone from 1925 wouldn't know a TV, but they might see a device in a room's central location and start to map it into being like a radio. They probably wouldn't recognize a cell phone but a land line (receiver on a hook) would be something they'd likely be able to take a guess at.
They might pick it up and ask for the operator, then believe it wasn't working when they heard the dial tone or got no response, but once they understood that device was called "phone" they have a ready understanding of what phones are used for, even if they didn't know specifically how to operate it. Cell phones, probably not so much, maybe if it received a call and rang like a phone, even if they didn't know how to touch the answer button they'd understand it was a phone and what could be done with it.
Someone from 1905 would get the buildings, the streets, probably the cars and inside a structure they'd understand things like the refrigerator, the toilet. They'd recognize the thing on the wall as a light and have an idea that if something, somewhere was operated, the light would turn on. They're likely to understand if they needed medicine it was likely in the room with the water fixtures and toilet.
The other devices would just be mysterious objects to them, wouldn't they?
Not that the person from 1905 couldn't discover or learn/be taught how to operate them, but they wouldn't have any initial mentel model for a television or an air conditioner, though they would understand the furnace.
Someone from 1885? I don't know.
I'd think a person from 1605 would be non-functional about where to find food, etc beyond a very basic level - they'd recognize people and buildings which would could act as shelter and could hold food, etc,, but everything else seems to me like it would be unbelievably incomprehensible to the point of being alien, wouldn't it?
I'm excluding language a little bit, as that's a whole complex topic unto itself - for example, a person from the state of Virginia in 1776 would recognize and probably read a map of the United States of America, but beyond proper nouns there would be many words, idioms and pronunciations that would be utterly foreign to them.
And the smells! We think of the ancient world as including the stink of open sewers and limited bathing and animals but think of what the smell of ozone and deodorant and car emissions (plus the absence of the scents they knew well would be like to an ancient person.
Not to mention the hum - the sound everyone living in electricity-using societies know and only notice when it's absent, like being in the country or during a power outage.
I also know this varies by era - if an Andaman Islander from 1605 dropped into the same place in the modern day he or she would be more likely to easily orient, but wondering what the cutoff would be for first/second world societies.
Electification is a big milestone, I know, but gas lamps were in homes for a long time before and I think someone of a late 1890s era would see the fixture, especially a clear one, and know what it did, whereas someone from James Fenimore Cooper's 1820s era might just see it as decoration.
At what point would the cognitive load of not -how- to operate modern technology but what it even did start to become so alien that the person would not be able to function?