The person i'm looking at attended school until 14, had a "normal" English education. Average intelligence, nothing special but far from stupid. Then, for the next 20 or so years, he's not really read anything except a clock, nor had to write anything.
How much is reading like riding a bike? If he was told to write his name now, at 34, could he? And what if he was given something to read - would he be able to? Of course he's been exposed to writing in the form of seeing roadsigns, or things in shops, but nothing that he's been actively trying to take notice of and process.