https://literacytrust.org.uk/parents-and-families/adult-literacy/what-do-adult-literacy-levels-mean/#:~:text=In%20England%2C%20adult%20literacy%20is,to%2011%2Dyear%2Dold.
I'm typing this only just after the first page of comments, so am no doubt only repeating what others have said.
The above is the English data where in 2011 they found that 1 in 7 adults in England had a literacy level at or below the 9-11 year age range.
In 2024, a different survey found that 1 in 6 adults had a literacy level below level 1 (ages 5-7).
Do those surveys look at the entire population, or just those with English as a First Language?
I work with many people that are all "adults in England", but hardly any of them come from the UK, so their literacy isn't always great, but it's a second language to them.
That said, my own DH's literacy skills are terrible. He cannot follow a recipe for cooking, for example, as he just misreads things.
He is, I think, part of a generation that skipped dyslexia assessment and just got dismissed as "stupid".
His own mum worked really hard with him to improve his reading skills from what they were and he had never believed he had any real issues with reading. He used to tell me he didn't like reading books because he was too slow a reader, but never thought that he actually had a genuine problem with reading.
That was, until our niece turned 7 and we took her to the zoo for the day and she read the signs outside the animal enclosures quicker than DH did.
It was at that point that he realised quite how poor his own reading skills were. Up until that point he thought I was just some sort of super reader, as opposed to just being able to read normally!
I still haven't managed to convince him to consider that he might be dyslexic, but I am going to try and show him something in a dyslexia font, to see if that makes a difference to him.
His writing is OK though - it's just his reading.
On the other hand, my Dad's writing is atrocious - poor spelling, no grammar, etc, but he loves reading.