(currently got a 'highly contagious' infection on my hands, so I'm typing with gloves on, on my phone, and can't be held responsible for any typos or typo-jokes, be they predictive-textly, intentional or otherwives)
I was born in 71 (that's old even with an apostrophe) and wet nightly up to about ten years old. As far back as I can remember of my childhood, I had polythene sheets on my bed which my dad made from industrial waste bags that he got from work.
They were mega noisy and I have a couple of tales on that side of things, but I can't be arsed typing, atm.
When I was about 10 or 11, I had to go into the sickhouse during the run up to Xmas and I do remember it being strange to sleep on an unprotected mattress; you'd have thought that they'd have had those fully covered mattresses, but the first I ever saw one of those was in the early noughties whilst on holiday.
(Attached to the tale of hospitalization is how I first publicly and purposely wet myself: I puked in class whilst sat in my chair and with my head facedown, thus getting some of the fluid on my lap and as the teacher had me stood me at the front of the class whilst waiting for the cleaner to come, and when she arrived he he said that I'd vomitted and added that I'd wet myself, too. I needed a piss by then, so I just let it go, partly in revenge for his saying that I'd wet myself in front of everyone).
I currently have a cheap plastic cover on my mattress, for all the reasons that have been mentioned in this thread. Unlike all my previous mattresses, it isn't a cheap, foam throwaway one nor a hand-me-down: I paid for it, it makes sense to protect it.